Myocardial adenyl cyclase: activation by thyroid hormones and evidence for two adenyl cyclase systems.

The mechanism responsible for the hyperdynamic circulatory state in hyperthyroidism has not been defined. Although certain cardiac manifestations resemble those caused by excessive adrenergic stimulation, recent evidence suggests that thyroid hormone exerts an effect on the heart that is independent of the adrenergic system. Since the inotropic and chronotropic effects of norepinephrine appear to be mediated by activation of adenyl cyclase, the possibility that thyroxine and triiodothyronine are also capable of activating adenyl cyclase was examined in the particulate fraction of cat heart homogenates.L-thyroxine and L-triiodothyronine increased the conversion of adenosine triphosphate-(32)P (ATP-(32)P) to cyclic 3',5'-adenosine monophosphate-(32)P (3',5'-AMP-(32)P) by 60 and 45% respectively (P < 0.01). A variety of compounds structurally related to the thyroid hormones, but devoid of thyromimetic activity did not activate adenyl cyclase: these included 3,5-diiodo-L-thyronine, L-thyronine, 3,5-diiodotyrosine, monoiodotyrosine, and tyrosine. D-thyroxine activated adenyl cyclase and half maximal activity was identical to that of the L-isomer. Although the beta adrenergic blocking agent propranolol abolished norepinephrine-induced activation of adenyl cyclase, it failed to alter activation caused by thyroxine. When maximal concentrations of L-thyroxine (5 x 10(-6) moles/liter) and norepinephrine (5 x 10(-5) moles/liter) were incubated together, an additive effect on cyclic 3',5'-AMP production resulted. THIS INVESTIGATION DEMONSTRATES: (a) thyroid hormone is capable of activating myocardial adenyl cyclase in vitro and (b) this effect is not mediated by the beta adrenergic receptor. Moreover, the additive effects of norepinephrine and thyroxine suggest that at least two separate adenyl cyclase systems are present in the heart, one responsive to norepinephrine, the other to thyroid hormone. These findings are compatible with the hypothesis that the cardiac manifestations of the hyperthyroid state may, in part, be caused by the direct activation of myocardial adenyl cyclase by thyroid hormone.

Activation of myocardial adenyl cyclase by histamine in guinea pig, cat, and human heart.

Histamine has positive inotropic and chronotropic effects on the heart which are not abolished by beta adrenergic-blocking agents. Since the positive inotropic and chronotropic effects of other hormones on the heart are thought to be mediated by cyclic 3',5'-AMP, we examined the effect of histamine on adenyl cyclase in particulate preparations of guinea pig, cat, and human myocardium. Histamine at the peak of its dose-response curve, 3 x 10(-4)moles/liter, produced approximately a 300% increase in cyclic 3',5'-AMP accumulation in the guinea pig, 60% in the cat, and 90% in the human heart particles. Half-maximal activity for the histamine mediated activation of adenyl cyclase in the guinea pig was 9 x 10(-6)moles/liter, almost identical with that observed for norepinephrine in the same preparation. DL-Propranolol, 1 x 10(-5)moles/liter, did not abolish the activation of adenyl cyclase produced by histamine but did abolish the activation produced by norepinephrine. In contrast, diphenhydramine hydrochloride, Benadryl, 8 x 10(-5)moles/liter, abolished the activation of adenyl cyclase by histamine but not that produced by norepinephrine. These data suggest that there are at least two receptor sites in guinea pig heart mediating the activation of adenyl cyclase, one responsive to histamine, the other to norepinephrine. In addition, combined maximal doses of histamine and norepinephrine produced completely additive effects on the activation of adenyl cyclase, which suggests that at least two separate adenyl cyclase systems are present in the heart, each responsive to one of these hormones. However, definitive proof would require physical separation of the two enzymes.

Solubilized myocardial adenylate cyclase. Restoration of histamine responsiveness by phosphatidylserine.

We have recently described the preparation of a solubilized cat myocardial adenylate cyclase which is unresponsive to histamine, norepinephrine, glucagon, and thyroxine, the hormones which activate the particulate enzyme. Since hormone receptors may consist of proteins and phospholipids, we determined the effect of several phospholipids on restoring the responsiveness of the solubilized adenylate cyclase to histamine. The addition of phosphatidylserine completely restored the histamine-mediated activation of the solubilized enzyme in contrast to phosphatidylethanolamine and phosphatidylinositol which were without effect. The concentration of histamine producing half-maximal activation of adenylate cyclase, 2 x 10(-5) M, was virtually identical with that observed in the particulate preparation. The antihistamine, diphenhydramine, 8 x 10(-5) M, abolished activation of adenylate cyclase by histamine in both the solubilized and particulate preparations. Phosphatidylserine also restored glucagon responsiveness, but did not restore norepinephrine responsiveness. It would appear that phosphatidylserine produced the necessary molecular configuration of the adenylate cyclase for histamine binding and activation of the enzyme.

Decreased myocardial adenyl cyclase activity in hypothyroidism.

It has been suggested that hypothyroidism may alter the responsiveness of the heart to sympathetic stimulation. To define more precisely the interrelationship between hypothyroidism and catecholamine responsiveness we: (a) studied the effects of norepinephrine and fluoride on the activation of adenyl cyclase in the particulate fraction of heart homogenates from euthyroid and hypothyroid cats; and (b) assessed the contractile response of isolated right ventricular papillary muscles from the same cats to increasing concentrations of norepinephrine. It was found that maximal accumulation of cyclic 3',5'-adenosine monophosphate (3',5'-AMP) was significantly lower at peak norepinephrine concentrations in the hypothyroid (284 +/-5 pmoles) than in the euthyroid group (326 +/-10 pmoles) (P < 0.02). However, the K(m) for norepinephrine was similar in both groups (1-2 x 10(-5) moles/liter), and there was no apparent change in the threshold concentration. Fluoride-mediated increases in Cyclic 3',5'-AMP accumulation were also significantly lower in the hypothyroid (585 +/-25 pmoles) as compared to the euthyroid group (790 +/-20 pmoles) (P < 0.02). In contrast, norepinephrine produced a similar augmentation of contractility in isolated papillary muscles from the hypothyroid and euthyroid cats. It thus appears that although the hypothyroid state is associated with a decrease in the total amount of myocardial adenyl cyclase per milligram of tissue capable of being activated by norepinephrine or fluoride, there is no change in the sensitivity of the enzyme to norepinephrine stimulation. Moreover, the finding that the inotropic response to norepinephrine is unaltered in hypothyroidism is compatible with the hypothesis that only a fraction of the total intracellular cyclic 3',5'-AMP produced by norepinephrine activation of adenyl cyclase is required to elicit the inotropic response.

Activation of renal cortical adenylate cyclase by circulating immunoreactive parathyroid hormone fragments.

Three distinct immunoreactive species of parathyroid hormone (PTH) are present in human serum. One has an estimated mol wt of 9,500 and probably represents glandular hormone, the second 7,000-7,500 mol wt, and the third 4,500-5,000 mol wt. In order to assess the biological activity of these circulating forms of PTH, we determined their ability to activate renal cortical adenylate cyclase. The 9,500 mol wt and 4,500-5,000 mol wt fractions produced four- to sixfold increases in cyclic 3',5'-AMP accumulation above control; the 7,000-7,500 mol wt fraction was inactive. None of the fragments had any effects on phosphodiesterase activity. Antiserum to bovine PTH did not block the activation of adenylate cyclase by either the gragments or bovine PTH. The data suggest that a large proportion of circulating immunoreactive human PTH is biologically active and that the biologically and immunologically active sites of the hormone are distinct.

The effect of sulfonylurea drugs on rabbit myocardial contractility, canine purkinje fiber automaticity, and adenyl cyclase activity from rabbit and human hearts.

Long-term clinical studies have associated tolbutamide therapy with an increased incidence of cardiovascular deaths. The effects of this and other sulfonylurea drugs on contractility and rate of isolated rabbit atria, automaticity of isolated dog Purkinje fibers, and adenyl cyclase activity in particulate preparations of rabbit and human hearts were studied. At concentrations that are attained clinically, tolbutamide (10 mg/100 ml) increased contractility of driven rabbit atria to 124+/-5% of control, acetohexamide (3.9 mg/100 ml) to 140+/-5%, chlorpropamide (8.3 mg/100 ml) to 139+/-6%, and tolazamide (3.1 mg/100 ml) to 119+/-6%. These effects were accentuated in the presence of 2.5 x 10(-4) M theophylline and were not blocked by 1 x 10(-5) M propranolol. Adenyl cyclase was activated by each of these drugs at concentrations below those which increase contractility. The drugs also increased the rate and slope of phase 4 depolarization in spontaneously beating Purkinje fibers, but did not alter the spontaneous rate of isolated rabbit atria. Since inotropic and chronotropic stimulation can be deleterious in some clinical settings, these findings may be of significance in interpretation of cardiovascular mortality data.

Effects of experimental heart failure on the capacity of glucagon to augment myocardial contractility and activate adenyl cyclase.

Although glucagon exerts positive inotropic effects in patients with no or mild impairment of cardiac function, similar effects are not consistently observed in patients with chronic heart failure. Accordingly, the inotropic effects of glucagon on papillary muscles from normal cats and cats in which right ventricular failure had been produced for 4-145 days by pulmonary artery banding were compared. At the peak of the concentration-response curve, glucagon increased peak isometric tension (T) in normal muscles from 4.4+/-0.4 to 6.6+/-0.5 g/mm(2) (P <0.001), and maximum rate of tension development (dT/dt) from 16.9+/-0.9 to 25.1+/-1.6 g/sec per mm(2) (P < 0.001). In contrast, glucagon produced no significant increases in T or dT/dt in failure muscles. The percentage increases in T and dT/dt caused by norepinephrine were the same in muscles from normal and failing hearts. Since the cardiac effects of glucagon and norepinephrine may be mediated by adenyl cyclase, responsiveness of adenyl cyclase was determined in particulate fractions of the right ventricle. Glucagon activated adenyl cyclase in normal, but had no effect in failure preparations. Norepinephrine-induced activation of adenyl cyclase, however, was unaltered by failure. Thus, in contrast to norepinephrine, glucagon loses the capacity to augment myocardial contractility and activate adenyl cyclase in hearts derived from cats in chronic failure.

Metabolism of bovine parathyroid hormone. Immunological and biological characteristics of fragments generated by liver perfusion.

The metabolism of bovine parathyroid hormone (PTH) by the perfused rat liver was studied. Labeled hormone, with or without cold hormone, was infused into the circulating perfusion medium containing various calcium concentrations. Pefusate samples at various time periods after the introduction of PTH into the system were chromatographed on Bio-gel P-10; radioactivity and/or immunoreactivity were measured in eluted fractions. Before the perfusion, all immuno- and radioactivity eluted in a single peak, with an apparent mol wt of 9,500 (peak I). After perfusion for 15 min, two other peaks with approximate mol wt of 7,000 (peak II) and 3,500 (peak III) were discernible. Peak I contained both NH2-terminal and COOH-terminal immunoreactivity and was biologically active at all time periods tested. The relative contribution of NH2-terminal and COOH-terminal immunoreactivity to the total immunoreactivity remained constant in this peak throughout the perfusion. In every respect, peak I had the characteristics of intact hormone. At all times, peak II consisted of only COOH-terminal immunoreactivity and was biologically inactive. At early time periods, peak III contained predominantly NH2-terminal immunoreactivity and was biologically active. With time, the relative contribution of NH2-terminal immunoreactivity decreased strikingly while that of COOH-terminal immunoreactivity increased. The three peaks identified in these experiments were analogous in size, biological activity, and immunological characteristics to those we have previously described for fractionated human hyperparathyroid serum. The rate of metabolism of PTH appeared to be regulated by the calcium concentration in the medium. At a high concentration of calcium (greater than 11 mg/100 ml), PTH metabolism was greatly retarded. At a low concentration of calcium (smaller than 5 mg/100 ml), the rate of metabolism was greatly increased. The physiological significance of our observations on the metabolism of PTH by isolated perfused rat liver is not known. However, since such metabolism results in a biologically active fragment, it is suggested that metabolism of intact hormone may be required before full biological expression is possible.

Activation of guanylate cyclase by streptozotocin and 1-methyl-1-nitrosourea.

Streptozotocin has been shown to induce the production of a variety of tumors in rats. The present report demonstrates that streptozotocin and 1-methyl-1-nitrosourea, a component of the streptozotocin molecule and a known carcinogen, stimulate the enzyme guanylate cyclase which catalyzes the production of guanosine 3',5'-monophosphate. At a maximal concentration of 3 mg/ml, these agents activated guanylate cyclase approximately 30-fold in liver, 20-fold in kidney, 15-fold in cerebellum. 15- to 30-fold in cerebrum, 4- to 20-fold inheart, 12-fold in brain stem, 10-fold in lung, and 2-fold in pancreas. Since recent evidence suggests a role for guanosine 3',5'-monophosphate in malignant transformation, the data may help explain the tumor-inducing capacity of these agents.

Cyclic AMP modulation of calcium accumulation by sarcoplasmic reticulum from fast skeletal muscle.

The role of cyclic 3',5'-AMP in modulating sarcoplasmic reticulum from fast skeletal muscle was studied. The rate of Ca2+ uptake was stimulated in the presence of protein kinase plus 1 micron cyclic AMP. The stimulation was absent when denatured protein kinase was used. When an adenylate cyclase inhibitor was added, the uptake rates fell to 55% of control. This decrease in rate was partially overcome by 1 micron cyclic AMP. A modulating role for cyclic AMP in fast skeletal muscle is proposed.

The cardiac sarcoplasmic reticulum-glycogenolytic complex. A possible effector site for cyclic AMP.

Cardiac sarcoplasmic reticulum-glycogenolytic complex, isolated as a single peak on sucrose density gradient, may function as a "compartmented" effector site for cyclic AMP resulting in modulation of both glycogenolysis and calcium transport. The conversion of phosphorylase b to a is stimulated by ATP and inhibited by protein kinase inhibitor. Cyclic AMP alone stimulated neither phosphorylase b to a conversion nor calcium uptake. An inhibitor of adenylate cyclase depressed both calcium uptake and phosphorylase activation and both functions were subsequently stimulated by micromolar concentrations of cyclic AMP. Endogenous phosphorylation of sarcoplasmic reticulum was also inhibited by adenylate cyclase inhibitor and the inhibition was reversed by cyclic AMP. These results suggest that the sarcoplasmic reticulum of cardiac muscle is an internal effector site for cyclic AMP which may regulate both calcium and metabolism. It appears that cyclic AMP formation in vitro is not the rate-controlling step in the activation sequence.

Decreased rat hepatic guanylate cyclase activity in streptozotocin-induced diabetes mellitus.

Guanylate cyclase is found in virtually all cells, but its physiologic role and the effect of hormones on its activity have not been clarified. Hepatic soluble guanylate cyclase activity (37,000 g supernatant) in rats with diabetes-mellitus-like syndrome induced by streptozotocin, 65 mg./kg. i.v., was 140 +/- 8 pmoles accumulated/mg. protein/10 min. (n = 13 rats) as against 279 +/- 16 pmoles accumulated/mg. protein/10 min. (n = 12 rats) in normal rats. The average blood sugar for the 12 normal rats was 100 +/- 4 mg./100 ml. and 546 +/- 32 mg./100 ml. for 13 diabetic rats. The decreased soluble hepatic guanylate cyclase activity in diabetic rats was completely restored to normal with 10 U. regular insulin, i.p. The maximum increase in guanylate cyclase activity was observed as early as five minutes and as late as two hours after insulin administration. Insulin restoration of guanylate cyclase was dose-related over a range of 1 U. to 10 U., i.p. Hepatic cyclic GMP levels in vivo paralleled in-vitro guanylate cyclase activity, being 29 +/- 0.4 pmoles/gm. wet weight in normals, 17 +/- 0.4 pmoles/gm. wet weight in streptozotocin-diabetic rats, and 38 +/- 0.4 pmoles/gm. wet weight two hours after the injection of 10 U. regular insulin. We conclude that rat hepatic guanylate cyclase is decreased in streptozotocin-induced diabetes and that insulin modulates this enzyme. The administration of exogenous insulin in normal animals did not further augment hepatic guanylate cyclase activity.

Adult cardiology and the expanding supply of physicians.

The number of cardiologists can be projected with considerable accuracy into the next century. The total cardiology pool of physicians will increase until the year 2015 at which time those entering and leaving the pool will come into equilibrium. At that time the ratio of active cardiologists to the population will have greatly increased. This nation's future need for cardiologists is difficult to assess with any degree of precision. Therefore, this is the time for updating practice profile studies. Such studies today could be formulated in a manner to provide more detailed information on the cardiologist's daily activities. In addition, a data base developed through methodology such as the consensus formation approach must be developed and updated on a periodic basis. Through such analyses it will be possible to quantitate the future needs of cardiovascular manpower.

Unusual manifestations of hypothyroidism.

Thyroid hormone exerts direct effects on essentially all of the organ systems of the body. Hypothyroidism is a frequently diagnosed endocrine disorder that has characteristic clinical signs and symptoms. In addition to these common manifestations, however, there are many additional manifestations of hypothyroidism that are less commonly acknowledged and include involvement of the hematologic, muscular, cardiac, and rheumatologic systems. It is important to recognize that these other organ systems may be involved and that the resulting disease states can dominate the clinical picture. As with the classic manifestations of hypothyroidism, these unusual manifestations respond to thyroid hormone replacement therapy. Thus, the importance of recognizing these signs and symptoms, as a result of hypothyroidism, is evident. This article emphasizes these less common manifestations of the patient with hypothyroidism, and, in addition, discusses the possible pathophysiologic mechanisms by which thyroid hormone deficiency can lead to organ system dysfunction.

Somogyi effect in patient with hypopituitarism.

A patient with diabetes mellitus and hypopituitarism developed the Somogyi effect that was characterized by insulin-induced hypoglycemia and rebound insulin-resistant hypoglycemia. This compensatory insulin-resistant hyperglycemia has generally been ascribed to the release of anterior hypophyseal hormones; however, our findings suggest that factors other than anterior hypophyseal hormones are involved.

Symptom rating scale for assessing hyperthyroidism.

A hyperthyroid symptom scale (HSS) was designed and administered to ten subjects with untreated Graves' disease. All subjects had clinical and chemical evidence of hyperthyroidism and reproducible HSS scores of 20 or more points. During sequential treatments with propranolol hydrochloride (phase 2) followed by propylthiouracil (phase 3) there was a significant decline in the HSS scores at each phase. Accompanying the decrease in HSS scores was a decrease in heart rate, but there was no change in thyroid function test results at phase 2 and a decrease in heart rate, thyroid function test results, and goiter size at phase 3. This new scale includes ten categories of symptoms, it is sensitive to changes in both the adrenergic and metabolic components of hyperthyroidism, and it is useful in the clinical assessment and management of patients with thyrotoxicosis.

Silent thyrotoxic thyroiditis in association with chronic adrenocortical insufficiency.

A 23-year-old woman had silent thyrotoxic thyroiditis and chronic adrenocortical insufficiency (Addison's disease). Recent evidence suggests that silent thyroiditis is an autoimmune disease characterized by lymphocytic infiltration of the thyroid and by transient hyperthyroidism, followed occasionally by transient hypothyroidism and eventual recovery. This case would seem to expand the spectrum of autoimmune thyroid diseases (Graves' disease, lymphocytic [Hashimoto's] thyroiditis, and hypothyroidism) associated with chronic adrenocortical insufficiency.

National Study of Internal Medicine Manpower. XIV: Patterns of residency and fellowship over time, 1987 update.

In response to concerns among internists following the 1987 internal medicine match, this report compares internal medicine trainees with those in other specialties since 1972, describes their paths through the internal medicine "pipeline," and documents their distribution and continuation rates in residency and subspecialty fellowship programs. It is based on the National Study of Internal Medicine Manpower, 1987-1988. Between 1972 and 1986 the number of trainees in internal medicine doubled, and the percentage of trainees in internal medicine grew from 20% to 25%, while the percentage in surgical specialties declined from 28% to 19%. The numbers of women and foreign medical school graduates training in internal medicine have continued to increase, but minority representation has stabilized. Women and minorities have lower continuation rates into fellowships, and distinctive patterns of subspecialization are found among women, minorities, and foreign medical school graduates.

Pseudopseudohypoparathyroidism and pheochromocytoma. Association or coincidence?

We saw a patient who had a pheochromocytoma producing hypertension along with clear evidence of pseudopseudohypoparathyroidism (PPHP). Although PPHP does not have the biochemical features of hypocalcemia and elevated parathyroid hormone levels as seen in pseudohypoparathyroidism, it seems from this case to share the potential for multiple endocrine neoplasia seen in a number of metabolic disorders in which pheochromocytoma may be a prominent manifestation.

Three-decade investigation of familial pheochromocytoma. An allele of von Hippel-Lindau disease?

BACKGROUND: Usually sporadic, pheochromocytoma can, on occasion, complicate genetic disorders, such as neurofibromatosis 1, von Hippel-Lindau disease, and multiple endocrine neoplasia 2; some families seem to have just pheochromocytoma, where it may have occurred by chance. The natural history of a large kindred believed to have an excess of pheochromocytoma 34 years ago was followed with the hypothesis that the predisposition was, in fact, present and that family education and surveillance would decrease mortality. METHODS: Prospective observation and diagnostic surveillance for pheochromocytoma were conducted on the inception cohort, defined as three branches of the kindred in 1960. Of 619 descendants of three (of 11) siblings of German origin, 333 were evaluated in person at least once in the three decades of surveillance. No pheochromocytomas were known to have occurred in the eight other branches. A total of 522 persons from the 11 branches were evaluated. RESULTS: Five of the eight initial patients with pheochromocytoma died of cardiovascular complications attributable to the tumor. In follow-up, eight additional relatives were newly diagnosed with pheochromocytomas (at an average age of 19 years), and others had additional or recurrent pheochromocytomas, meningioma, para-adrenal paraganglioma, and a functioning glomus vagale; none died. CONCLUSIONS: A continuing excess of pheochromocytoma seems present in the family. Whether the incompletely penetrant gene in this family is allelic to the von Hippel-Lindau gene on chromosome 3 or is a distinct locus remains to be resolved with molecular studies. Meanwhile, education and surveillance seem to decrease mortality from pheochromocytoma in this family.