Before identifying means to treat acromegaly, ... GH-secreting pituitary tumors and acromegaly. ..... in infancy: a survey of possible disorders and guidelines for.
Increased Serum Growth Hormone Concentration in Feline Hypertrophic Cardiomyopathy Mark D. Kittleson, DVM, PhD, Paul D. Pion, DVM, Laura A. DeLellis, DVM, Yousry Mekhamer, BVSc, Noel Dybdal, DVM, PhD, and Clinton D. Lothrop, Jr., DVM, PhD
Serum growth hormone concentration was measured by radioimmunoassay in 31 cats with hypertrophic cardiomyopathy, 38 normal cats, and 35 cats with other cardiac disease. Cats with hypertrophic cardiomyopathy had a significantly increased serum growth hormone concentration when compared with normal cats and cats with other cardiac disease. The serum growth hormone concentration in cats with hypertrophic cardiomyopathy was less than that previously reported in cats with growth hormone secreting pituitary tumors. Pituitary tumors were not identified in eight of the cats with hypertrophic cardiomyopathy examined at necropsy. An increased serum growth hormone concentration may be measured in cats with hypertrophic cardiomyopathy but it is unclear if the increased serum growth hormone concentration is a cause or effect of hypertrophic cardiomyopathy. (Journal of Veterinary Internal Medicine 1992; 6~320-324)
HYPERTROPHIC CARDIOMYOPATHY is characterized by primary concentric hypertrophy of the left ventricular myocardium. The concentric hypertrophy is considered primary and inappropriate because it does not develop secondary to increased systolic intraventricular pressure. Hypertrophic cardiomyopathy has been identified in several species including people and dogs but appears to be especially prevalent in cats.' In people, several forms are described including asymmetric septa1hypertrophy, concentric hypertrophy of both the interventricular septum and left ventricular free wall, and apical hypertrophic cardiomyopathy.' In cats, the most common form is concentric hypertrophy of both
From the Departments of Medicine (Kittleson, Pion, DeLellis, and Mekhamer) and Pathology (Dybdal), University of California, Davis, California, and the Department of Environmental Practice (Lothrop), University of Tennessee, Knoxville, Tennessee. Supported by a grant from the Moms Animal Foundation. The authors thank Grant C. Knowlen, DVM, PhD, Janice M. Bright, DVM, and Mark E. Peterson, DVM, for submitting serum samples from affected cases, Kathi Greene and Jan Lansing for helping with technical assistance, and James L. Carpenter, DVM, for determining normal pituitary weights in cats. Accepted for publication July 15, 199I . Reprint requests: Mark D. Kittleson, DVM, PhD, Department of Medicine, School of Veterinary Medicine, University of California, Davis, CA 956 16.
the interventricular septum and the free wall of the left ventricle, although, asymmetric hypertrophy ofthe interventricular septum or left ventricular free wall are occasionally identified. The papillary muscles are usually severely thi~kened.~ In people, the disease can be familial and has also been associated with a number of inborn errors of metaboli~m.~*~ However in many cases the cause of the disease is unknown. Excess growth hormone (GH) secretion in mature individuals causes acromegaly. People with acromegaly develop concentric hypertrophy of the left ventricle. The amount of hypertrophy correlates with the duration of the increase in serum GH concentration6and the hypertrophy may regress after successfultreatment of the acromegaly.' Before identifying means to treat acromegaly, cardiac disease was the most common terminal event in human patienk8 In people, acromegaly is usually caused by a pituitary tumor.' A small number of cats has been described with GH-secreting pituitary tumors and acromegaly." Cats with pituitary tumors and acromegaly commonly have concentric symmetric or asymmetric hypertrophy of the left ventricular myocardium and heart failure. lo This study determined whether serum GH concentration was increased in cats with hypertrophic cardiomyopathy and without clinical signs of acromegaly.
320
Vol. 6
*
32 1
FELINE HYPERTROPHIC CARDIOMYOPATHY
No. 6. 1992
Materials and Methods
Patient Population and Sample Collection Cats with hypertrophic cardiomyopathy were examined from referral veterinary clinics at the University of California-Davis (19 cats), the University of Tennessee (3 cats), the Animal Medical Center (7 cats), and Washington State University (2 cats). For this study, hypertrophic cardiomyopathy was defined as symmetric or asymmetric concentric hypertrophy of the left ventricular free wall and/or interventricular septum to >6 mm in diastole along with no evidence of aortic stenosis, hyperthyroidism, or renal failure. Measurement of left ventricular wall thicknesses was performed using M-mode echocardiography. In some situations in which this was difficult because of the amount of hypertrophy, the wall thickness measurements were taken from a diastolic frame on the two-dimensional echocardiogram. Systemic arterial blood pressure measurement and histopathology to exclude hypertension and infiltrative diseases of the left ventricular myocardium were not routinely performed. At the time of diagnosis, blood was obtained from the jugular vein for measurement of serum GH and thyroxine (T4)concentrations and evaluation of a serum chemistry panel. Cats were not resting and no attempt was made to obtain fasting samples or to obtain samples at a uniform time of day. Samples were obtained more than once in some cats. Blood samples were also obtained for measurement of serum GH and T, concentrations from 38 normal cats and 35 cats with other cardiac diseases. The cats with cardiac disease had numerous different diseases including dilated cardiomyopathy (with and without heart failure), intermediate cardiomyopathy, mitral regurgitation, and congenital shunting defects. Intravenous glucose tolerance tests were performed in six catswith HCM by administering0.5 glkgglucose intravenously and taking samples for blood glucose and serum insulin concentrations immediately before and 10, 20, 30,45, and 60 minutes after glucose administration. Cats that died and were available for necropsy had their pituitary glands removed and weighed. Gland weights were compared with those from a group of ten normal cats. The glands were sectioned at 5 pm intervals, and every other slice was stained and examined for presence of tumor.
Hormone Analysis Serum GH concentration was measured using a heterologous canine radioimmunoassay (RIA) that has been previously validated for feline GH." Serum T4and insulin concentrationswere determined by RIA.
Statistical Analysis Data are presented as box plots and notched box plots. These plots depict the median, the 95% confidence inter-
val around the median (the waists on some of the plots), upper and lower quartiles about the median, and the outer quartiles." The growth hormone data were not normally distributed. Therefore comparisons between groups were performed using nonparametric analysis (Kruskal-Wallistest with multiple comparisons).'2Some cats, including normal cats, had serum analyzed for GH more than once. When a cat had its GH concentration measured more than once, an average value was used to represent a cat's GH status for statistical analysis. Results Normal serum GH concentration in the cat has been previously defined with the assay used in this study to be 1.21 f 1.O ng/mL (mean & SD)(range 0-8.5 ng/mL).'O The box plot in Figure 1 depicts the normal population from the present study. The upper value of the second quartile above the median was defined as the upper limit for normal serum GH concentrations. This made 1 1 ng/ mL the upper limit of normal serum GH concentration in the present population of cats. There were 5 of 38 (13%) normal cats with serum GH concentrations .greaterthan this upper limit (statistical outliers) when an averagevalue was used when instead of using the average value to represent a GH status of a cat, the highest value obtained was used, the upper limit for normal values was 12 ng/mL. Using this criterion, 7 of 38 (18%) normal cats had an increase in serum GH concentration. Notched box plots of the GH data from each group of cats are presented in Figure 2. Serum GH concentration was significantly (P< 0.00 1) greater in the sample popu-
20
*
15
-
n
I4
E
\
g
v
10-
5 5 -
0-
*
a
FIG. 1. A box plot of values for serum growth hormone concentration from a sample population of 38 normal cats. The median value is representedas the horizontal line in the box. The box representsquartile values on either side of the median. The vertical line above the box representsthe upper quartile. * outliers. The * at 15 ng/mL represents four values.
322
Journal of Veterinary Internal Medicine
KllTLESON ET AL.
8o
r--l ncm
NORMAL
OCD
DISEASE
FIG.2. Notched box plots of values for serum growth hormone concentration from a group of normal cats (n = 38), a group of cats with other cardiac disease (n = 35), and a group of cats with hypertrophiccardiomyopathy (n = 3 I). The box plots are as in Figure 1 except “waists” representing the 95% confidence interval about the median are d i s played. HCM = hypertrophic cardiomyopathy.OCD = other cardiac disease. The * at I5 ng/mL represents four values. * and o = outliers.
lation of cats with HCM than in the normal cats and the cats with other cardiac disease. Serum GH concentration in the group of cats with other cardiac diseases was not significantly different from that identified in the normal group of cats. Of the cats with hypertrophic cardiomyopathy, six had samples taken on more.than one occasion. Of these six cats, two had two samples taken and both were > 1 1 ng/ mL. One cat had two samples taken and both were 12 ng/mL) at any time is considered a positive test, then of the 3 1 cats with HCM tested, 19 (6 1%) had an increased serum GH concentration. If one averages the values for each cat, then GH concentration in 19 of 3 1 (61%)cats again was increased (>1 1 ng/mL). In the cats with other cardiac diseases, if a serum GH concentration > 12 ng/mL at any time is considered a positive test, then 6 of the 3 1 cats (19%)with other cardiac diseases had increases in serum GH concentration. If all values from an individual cat are averaged, then 5 of the 3 1 ( 16%)cats with other cardiac disease had increases in serum GH concentration. Serum GH concentration was not different in cats with heart failure or cats without heart failure. This included cats with HCM and cats with other cardiac diseases, including dilated cardiomyopathy. There were no overt diabetic cats in any group. Five cats with HCM and increased serum GH concentration underwent glucosetolerance tests; three had prediabetic glucose tolerance curve^.'^ One cat with HCM and a normal serum GH concentration had a prediabetic glucose
tolerance curve. Serum T4 concentration was not increased in any cat in this study. Pituitary glands from ten normal cats were removed and weighed. Weight ranged from 35 to 70 mg (52 -t 1 1 mg). Eight cats that died from complications of hypertrophic cardiomyopathyhad their pituitary glands examined. Four of these eight cats had increased serum GH concentration. There was no significant increase in pituitary weight in any cat with HCM when compared with the group of ten normal cats and tumors were not identified in these glands. Discussion In this study, serum was obtained from cats for the measurement of serum GH concentration using an assay developed for the determination of serum GH concentration in dogs but which has been previously validated for measuring serum GH concentration in the cat.” The cats with hypertrophic cardiomyopathy had significantly increased serum concentration of GH when compared with normal cats and cats with other cardiovascular diseases. Of the cats with HCM, 6 1% had increased serum GH concentration. The increase observed was mild when compared with cats with acromegaly due to a functional pituitary tumor (mean 26; range 1-101 ng/mL for HCM vs. mean 69; range 22- 131 ng/mL for cats with acromegaly).” The fact that the population of cats with HCM had serum GH concentrations greater than cats with other cardiac disease, some of which were in heart failure, demonstrated that cardiac disease and heart failure by themselves were not responsible for the increased serum GH concentration. Because pituitary tumors were not identified in the cats examined in this study, the increase in the serum GH concentration was more likely secondary to some other abnormality. However, pituitary tumors cannot be excluded in the cats whose pituitary glands were not examined. The regulation of GH secretion is complex and not completely understood in all speciesstudied. Regulation also appears to be different in the cat when compared with other species. In people and monkeys, insulin-induced hypoglycemia, arginine infusion, 2-deoxyglucose infusion, and fasting increase serum GH concentration whereas intravenous glucose infusion decreases it.9 In one study in cats, of the aforementioned factors only intravenous 2-deoxyglucose infusion significantly increased serum GH c~ncentration.’~ Growth hormone secretion in all speciesstudied is episodi~.’~-’’ Therefore serum GH concentration can be increased in a normal individual if taken when secretion is increased. To circumvent this problem in people, blood samples are taken in the morning, before food intake, before ambulation, and preferably immediately after awakening. If an increased serum GH concentration is identified in a human patient, glucose may be adminis-
Vol. 6
. NO. 6, 1992
323
FELINE HYPERTROPHIC CARDIOMYOPATHY
tered intravenously to determine if the serum GH concentration will decrease (normal subject) or not (acromegalic ~atient).~.’ Since intravenous glucose administration does not suppress serum GH concentration in the cat, this test is not useful and was not attempted in this study. The other test used to confirm the diagnosis of acromegaly in people is measurement of serum somatomedin-C (SMC) or insulin-like growth factor-I. Serum SMC was not measured in the cats in this study because an assay for feline SMC has not been validated. Spikes in serum GH concentration appear to occur in the cat. This is based on the observation that some normal cats had serum GH concentrations above the normally defined range, although this could be due to assay inaccuracy. Because of this finding, an increased serum GH concentration in an individual cat must be interpreted with caution. As a sample population, cats with HCM have an increase in serum GH concentration. To make determinations in individual cats, a method of suppressing GH release in the cat must be identified or a method of obtaining a sample from a recently awakened, unstressed cat must be found or multiple determinations of serum GH concentration must be made over a 24hour period. If serum GH concentration is increased in cats with HCM and it is not due to pituitary tumors, another factor must be present stimulating its release. Speculation regarding the type of stimulation present is difficult because of the complex regulation of GH secretion and because regulation in the cat does not appear to be the same as in other species. GH secretion is regulated by GH-releasing hormone (GHRH) and somatostatin. Numerous factors impact the release and inhibition of these two regulatory hormones. Most studies have been done in people and rats. A recent model has been proposed in which alpha-adrenergic stimulation, GABA, acetylcholine, and enkephalins stimulate GHRH release and increase circulating GH concentration. Beta-adrenergic stimulation in this model stimulates somatostatin release and decreases serum GH concentration. Dopamine stimulates release of both GHRH and somatostatin.” One could speculate that the autonomic nervous systemtould be abnormal in cats with HCM resulting in a secondary increase in GH. Central injection of norepinephrine does increase serum GH concentration in the cat.” Peripheral infusion of L-dopa also stimulates GH release in the cat.’O Administration of phenoxybenzamine, an a-blocker, abolishes the release of GH after L-dopa administration.20An increase in epinephrine or norepinephrine concentration has been reported previously in cats with HCM.’l Whether these increases were the cause or the effect of the disease and/or heart failure was not determined. A primary deficiency in somatostatin could also cause the increase in serum growth hormone concentration. This is attractive since in other specieswhen antisomato-
statin antibodies are administered, serum GH concentration increases to a level similar to that observed in the present study.22An increase in serum GABA concentration could also cause an increase in serum GH concent r a t i ~ nWe . ~ ~have examined serum GABA concentration in five cats with HCM and found one cat with a marked increase. However, the other four cats did not have an increase above that observed in a control population. Galanin, another neurosecretory peptide, increases GH secretion, possibly dire~tly.’~ In people, hypertrophic cardiomyopathy can be familial.5 Transgenic mammals can be produced that elaborate increased GH or GHRH.25In one situation, genes encoded for producing human or bovine GH or GHRH have been introduced into the germ line of pigsz5Subsequently, lines of pigs have been produced that have increased serum GH concentration.Serum GH concentration has varied among different lines but has been consistent within a line. These transgenic pigs have not had an increase in long bone growth. Organomegaly, including cardiomegaly, increased long bone circumference, and improved feed efficiency have been identified. Because the control of GH secretion is so complex, a number of possible explanations exist for the increase in serum GH concentration observed in this study. The factor or factors responsible for the increase in serum GH concentration could be responsible for the cardiac growth observed in HCM. Conversely, growth hormone itself could be responsible for the hypertrophy or growth hormone or its modulators could have nothing to do with HCM or their increase could be caused by HCM. Concentric left ventricular hypertrophy in the cat can be caused by aortic stenosis, systemic hypertension, hypertrophic cardiomyopathy, or infiltrative diseases of the left ventricular myocardium (e.g., lymphoma).26Fixed aortic stenosiswas excluded as a possible cause in all cats in this study by auscultation and echocardiography. Infiltrative diseases of the myocardium could not be excluded although no cat included in the study had clinical evidence of lymphoma in other organs. Systemic hypertension was not excluded in all cats because systemic arterial blood pressure is difficult to obtain in the cat. Several cats did have limb-cuff Doppler determinations of systolic arterial blood pressure less than
