Epidermal growth factor stimulates cAMP accumulation in cultured rat cardiac myocytes

Epidermal growth factor protects the heart against low-flow ischemia-induced injury

The role of ErbB4 and ErbB2 in the heart of adult mammals is well established. The heart also expresses ErbB1 (the epidermal growth factor (EGF) receptor), but this receptor has received less attention. We studied the effect of EGF on the response of isolated mouse heart to low-flow ischemia and reperfusion. Reducing perfusate flow to 10% for 30 min resulted in an increase in anaerobic metabolism and the leakage of lactate dehydrogenase during reperfusion. In addition, left ventricle +dP/dt and developed pressure were depressed (20-25%) during reperfusion. The addition of EGF 5 min before and throughout the ischemic period prevented the increase in anaerobic metabolism and the leakage of intracellular lactate dehydrogenase during reperfusion. EGF improved both +dP/dt and developed pressure during ischemia and prevented the decrease in dP/dt during reperfusion. To determine whether the effect of EGF on cell integrity depends on its effect on contractility, we studied nonbeating isolated myocytes. In these cells, anoxia and reoxygenation reduced cell viability by nearly 25%. EGF prevented such a decrease. Our results indicate that, like ErbB4 and ErbB2, ErbB1 also has an important role in the heart of adult animals.

Acute and chronic adrenergic stimulation of submandibular salivary glands. Effects on the endocrine function of epidermal growth factor in mice

Submandibular salivary glands are the major source of epidermal growth factor (EGF) in mice. Acute secretion of EGF from these glands protects the heart against catecholamine-induced injury. Little is known about chronic adrenergic stimulation of salivary glands and the contribution of accumulated EGF to the adaptive hypertrophic response of the heart to such chronic adrenergic stimulation. Here we show that the EGF content of submandibular glands did not recover to normal values 24 h after a single phenylephrine injection or an aggressive encounter. Repeated (twice a day for 2 days) adrenergic stimulation resulted in an almost 90% decrease in EGF content in the submandibular glands. In these conditions, new adrenergic stimulation did not result in an increase in plasma EGF concentration, or in the activation of liver ErbB1 (the EGF receptor). Chronic isoproterenol or phenylephrine administration (7 days) induced atrial natriuretic factor expression in the heart and an increase in both ventricular weight and protein. The surgical removal of submandibular glands (sialoadenectomy) did not affect these adaptive responses of the heart. We conclude that EGF from submandibular glands does not contribute to heart hypertrophy, one of the adaptive responses induced by chronic adrenergic stimulation.

ErbB receptors protect the perfused heart against injury induced by epinephrine combined with low-flow ischemia

ErbB receptor tyrosine kinases are important in maintaining the long-term structural integrity of the heart and in the induction of hypertrophy. In addition, in vivo activation of ErbB1 by epidermal growth factor (EGF) protects the heart against acute stress-induced damage. We examined here whether the ErbB sytem acutely protects the isolated heart in which stress was induced in vitro by ischemia combined with epinephrine infusion (EPI). In perfused mouse hearts, EGF induced Tyr-phosphorylation of ErbB1 but not ErbB2. Neuregulin-1beta (NRG-1beta) induced Tyr-phosphorylation of both ErbB4 and ErbB2. We also found differences in the signaling cascades activated by each growth factor. To stress the perfused mouse heart, we combined EPI with low-flow ischemia. This resulted in (i) loss of left ventricle contraction force ( + dP/dt(max)) and developed pressure (LVDP) after a short period of hypercontractility, (ii) enhanced anaerobic metabolism (lactate production), and (iii) myocyte injury (lactate dehydrogenase (LDH) release). EGF and NRG-1beta had different effects on stressed-heart contractility. EGF reduced to a half the loss of both + dP/dt(max) and LVDP. In contrast, NRG-1beta exacerbated the hypercontractility soon after reperfusion. This is coincident with a transient increase in coronary flow after reperfusion. In spite of these differences in contraction, both EGF and NRG-1beta induced similar early protection as shown by the reduction of LDH release. Our results show that the ErbB system protects the perfused heart against damage induced by acute stress. They reinforce the relevance of ErbB receptors and ligands in cardiac physiology.

Physiological roles for G protein-regulated adenylyl cyclase isoforms: insights from knockout and overexpression studies

Cyclic AMP is a universal second messenger, produced by a family of adenylyl cyclase (AC) enzymes. The last three decades have brought a wealth of new information about the regulation of cyclic AMP production by ACs. Nine hormone-sensitive, membrane-bound AC isoforms have been identified in addition to a tenth isoform that lacks membrane spans and more closely resembles the cyanobacterial AC enzymes. New model systems for purifying and characterizing the catalytic domains of AC have led to the crystal structure of these domains and the mapping of numerous interaction sites. However, big hurdles remain in unraveling the roles of individual AC isoforms and their regulation in physiological systems. In this review we explore the latest on AC knockout and overexpression studies to better understand the roles of G protein regulation of ACs in the brain, olfactory bulb, and heart.

Insensitivity of cardiac delayed-rectifier I(Kr) to tyrosine phosphorylation inhibitors and stimulators

1. The rapidly activating delayed-rectifying K+ current (I(Kr)) in heart cells is an important determinant of repolarisation, and decreases in its density are implicated in acquired and inherited long QT syndromes. The objective of the present study on I(Kr) in guinea-pig ventricular myocytes was to evaluate whether the current is acutely regulated by tyrosine phosphorylation. 2. Myocytes configured for ruptured-patch or perforated-patch voltage-clamp were depolarised with 200-ms steps to 0 mV for measurement of I(Kr) tail amplitude on repolarisations to -40 mV. 3. I(Kr) in both ruptured-patch and perforated-patch myocytes was only moderately (14-20%) decreased by 100 microM concentrations of protein tyrosine kinase (PTK) inhibitors tyrphostin A23, tyrphostin A25, and genistein. However, similar-sized decreases were induced by PTK-inactive analogues tyrphostin A1 and daidzein, suggesting that they were unrelated to inhibition of PTK. 4. Ruptured-patch and perforated-patch myocytes were also treated with promoters of tyrosine phosphorylation, including phosphotyrosyl phosphatase (PTP) inhibitor orthovanadate, exogenous c-Src PTK, and four receptor PTK activators (insulin, insulin-like growth factor-1, epidermal growth factor, and basic fibroblast growth factor). None of these treatments had a significant effect on the amplitude of I(Kr). 5. We conclude that Kr channels in guinea-pig ventricular myocytes are unlikely to be regulated by PTK and PTP.

Regulatory properties of adenylate cyclases type 5 and 6: A progress report

Adenylate cyclases (AC) type 5 and 6 comprise the calcium-inhibited family of adenylate cyclase isoforms. Here we review recent discoveries in the regulation of AC5 and AC6 with a focus on posttranslational modifications including glycosylation, nitrosylation, and phosphorylation by the cyclic AMP-dependent protein kinase (PKA), protein kinase C (PKC), and Raf1. We also describe novel signaling interactions such as Galpha(q)-mediated potentiation of AC6 activation. Novel regulators of AC5 and AC6, including small molecules and proteins that physically interact with AC5 and AC6 such as snapin, regulator of G protein signaling 2 (RGS2), protein associated with myc (PAM), and caveolin peptides are discussed. We also describe several recent studies that demonstrate the usefulness of transgenic or adenoviral overexpression of AC5 and AC6 in models for disease states such as cardiovascular hypertrophy. The discovery of novel regulatory mechanisms for AC5 and AC6 and their potential role in crucial physiological processes provide new avenues for research into therapeutic interventions targeting the cyclic AMP pathway.

Protein kinase C and epidermal growth factor stimulation of Raf1 potentiates adenylyl cyclase type 6 activation in intact cells

Adenylyl cyclase type 6 (AC6) activity is inhibited by protein kinase C (PKC) in vitro; however, in intact cells, PKC activation does not inhibit the activity of transiently expressed AC6. To investigate the effects of PKC activation on AC6 activity in intact cells, we constructed human embryonic kidney (HEK) 293 cells that stably express wild-type AC6 (AC6-WT) or an AC6 mutant lacking a PKC and cyclic AMP-dependent protein kinase (PKA) phosphorylation site, Ser674 (AC6-S674A). In contrast to in vitro observations, we observed a PKC-mediated enhancement of forskolin- and isoproterenol-stimulated cyclic AMP accumulation in HEK-AC6 cells. Phorbol 12-myristate 13-acetate also potentiated cyclic AMP accumulation in cells expressing endogenous AC6, including Chinese hamster ovary cells and differentiated Cath.a differentiated cells. In HEK-AC6-S674A cells, the potentiation of AC6 stimulation was significantly greater than in cells expressing AC6-WT. The positive effect of PKC activation on AC6 activity seemed to involve Raf1 kinase because the Raf1 inhibitor 3-(3,5-dibromo-4-hydroxybenzylidene-5-iodo-1,3-dihydro-indol-2-one (GW5074) inhibited the PKC potentiation of AC6 activity. Furthermore, the forskolin-stimulated activity of a recombinant AC6 in which the putative Raf1 regulatory sites have been eliminated was not potentiated by activation of PKC. The ability of Raf1 to regulate AC6 may involve a direct interaction because AC6 and a constitutively active Raf1 construct were coimmunoprecipitated. In addition, we report that epidermal growth factor receptor activation also enhances AC6 signaling in a Raf1-dependent manner. These data suggest that Raf1 potentiates drug-stimulated cyclic AMP accumulation in cells expressing AC6 after activation of multiple signaling pathways.

Single Transmembrane Spanning Heterotrimeric G Protein-Coupled Receptors and Their Signaling Cascades

Heptahelical of serpentine receptors such as the adrenergic receptors are well known to mediate their actions via heterotrimeric GTP-binding proteins. Likewise, receptors that traverse the cell membrane once have been shown to mediate their biological actions by activating several different mechanisms including stimulation of their intrinsic tyrosine kinase activities or the kinase activities of other proteins. Some of these single transmembrane receptors have an intrinsic guanylyl cyclase activity and can stimulate the cyclic GMP second messenger system; however, over the last few years, several studies have shown the involvement of heterotrimeric GTP-binding proteins in mediating signals that eventually culminate in the biological actions of single transmembrane spanning receptors and proteins. These receptors include the receptor tyrosine kinases that mediate the actions of growth factors such as epidermal growth factor, insulin, insulin-like growth factor as well as receptors for atrial natiuretic hormone or the zona pellucida protein (ZP3) and integrins. In this review, the significance of the coupling of the single transmembrane spanning receptors to G proteins has been highlighted by providing several examples of the concept that signaling via these receptors may involve the activation of multiple signaling cascades.

Effects of epidermal growth factor on epinephrine-stimulated heart function in rodents

Epidermal growth factor (EGF) interferes with beta-adrenergic receptor (beta-AR) signaling in adipocytes and hepatocytes, which leads to decreased lipolytic and glycogenolytic responses, respectively. We studied the effect of EGF on the heart. EGF interfered with the cAMP signal generated by beta-AR agonists in cardiac myocytes. In perfused hearts, EGF decreased inotropic and chronotropic responses to epinephrine but not to 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate. Sustained epinephrine infusion induced heart contracture, which resulted in altered heart function as demonstrated by decreased inotropy and increased heart rate variability. EGF prevented all these alterations. In the whole animal (anesthetized mice), EGF administration reduced the rise in heart rate induced by a single epinephrine dose and the occurrence of Bezold-Jarisch reflex episodes induced by repeated doses. Sialoadenectomy enhanced the response to epinephrine, and EGF administration restored normal response. All these results suggest that, by interfering with beta-AR signaling, EGF protects the heart against the harmful effects of epinephrine.

The inhibitory gamma subunit of the type 6 retinal cyclic guanosine monophosphate phosphodiesterase is a novel intermediate regulating p42/p44 mitogen-activated protein kinase signaling in human embryonic kidney 293 cells

The inhibitory gamma subunits of the retinal rod and cone photoreceptor type 6 retinal cyclic guanosine monophosphate phosphodiesterase (PDEgamma) are expressed in non-retinal tissues. Here, we show that PDEgamma interacts with the G-protein-coupled receptor kinase 2 signaling system to regulate the epidermal growth factor- and thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase in human embryonic kidney 293 cells. This is based upon several lines of evidence. First, the transfection of cells with an antisense rod PDEgamma plasmid construct, which reduced endogenous rod PDEgamma expression, ablated the epidermal growth factor- and thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase. Second, the transfection of cells with recombinant rod or cone PDEgamma and/or G-protein-coupled receptor kinase 2 increased the stimulation of p42/p44 mitogen-activated protein kinase by epidermal growth factor or thrombin. In contrast, a G-protein-coupled receptor kinase 2 phosphorylation-resistant rod PDEgamma mutant failed to increase the epidermal growth factor- or thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase and, in fact, functioned as a dominant negative. Thrombin also stimulated the association of endogenous rod PDEgamma with dynamin II, which was increased in cells transfected with rod PDEgamma or G-protein-coupled receptor kinase 2. Dynamin II plays a critical role in regulating endocytosis of receptor signal complexes required for activation of p42/p44 mitogen-activated protein kinase. Therefore, PDEgamma may have an important role in promoting endocytosis of receptor signal complexes leading to the activation of p42/p44 mitogen-activated protein kinase. We conclude that PDEgamma is an entirely novel intermediate regulating mitogenic signaling from both receptor tyrosine kinase and G-protein-coupled receptors in human embryonic kidney 293 cells.

The adaptive intestinal response to massive enterectomy is preserved in c-SRC-deficient mice

BACKGROUND/PURPOSE

The Src family of protein tyrosine kinases has been implicated in the downstream mitogenic signaling of several ligands including epidermal growth factor (EGF). Because EGF likely plays a role in adaptation after massive small bowel resection (SBR), we tested the hypothesis that c-src is required for this important response.

METHODS

A 50% proximal SBR or sham operation (bowel transection or reanastomosis alone) was performed on c-src-deficient (n = 14) or wild-type (C57bl/6) mice (n = 20). The ileum was harvested on postoperative day 3 and adaptive parameters determined as changes in ileal wet weight, protein and DNA content, proliferation index, villus height, and crypt depth. Comparisons were done using analysis of variance (ANOVA), and a Pvalue less than .05 was considered significant. Values are presented as mean +/- SEM.

RESULTS

The activity of c-src was increased in the ileum of wild-type mice after SBR but remained unchanged in c-src-deficient mice. Despite this lack of increase, adaptation occurred after SBR in the c-src-deficient mice as demonstrated by increased ileal wet weight, protein and DNA content, proliferation index, villus height, and crypt depth similar to wild-type mice.

CONCLUSIONS

The adaptive response of the intestine to massive SBR is preserved despite reduced activity of the c-src protein. The mitogenic signaling that characterizes intestinal adaptation and is associated with receptor activation by EGF or other growth factors probably occurs by mechanisms independent of c-src protein tyrosine kinase.

Modulation of cyclic AMP levels in a clonal neural cell line by inhibitors of tyrosine phosphorylation

The convergence of tyrosine kinase and cyclic AMP (cAMP) signal transduction pathways was investigated in the HT4.7 neural cell line with inhibitors of tyrosine kinases and tyrosine phosphatases. The protein tyrosine kinase inhibitor genistein inhibited isoproterenol-stimulated cAMP production by 40-60% in whole cells, with no effect on basal cAMP levels. In both whole cells and membranes, genistein also inhibited cAMP produced in response to direct stimulation of adenylyl cyclase with forskolin. However, in the absence of phosphodiesterase inhibitors, genistein presentation resulted in an increase in cAMP levels. Genistein inhibited phosphodiesterase activity by 80-85%, indicating that tyrosine phosphorylation stimulates both cAMP synthesis and degradation. The decrease in cAMP levels by genistein was not merely competitive inhibition of adenylyl cyclase with respect to ATP, since the Km of adenylyl cyclase for ATP remained essentially the same in either the presence or the absence of genistein. Another tyrosine kinase inhibitor, herbimycin A, which inhibits by a different mechanism than genistein, also decreased forskolin-stimulated cAMP in whole cells. As would be expected for the involvement of tyrosine phosphorylation in the control of cAMP production, inhibition of tyrosine phosphatases by vandate increased forskolin-stimulated cAMP production. These results suggest that cAMP production can be regulated by tyrosine phosphorylation, and the simultaneous activation of both cAMP synthesis and degradation may serve to alter the duration of cAMP elevation.

Expression of type V adenylyl cyclase is required for epidermal growth factor-mediated stimulation of cAMP accumulation

Previously, this laboratory has demonstrated that epidermal growth factor (EGF) increases adenylyl cyclase activity in cardiac membranes and elevates cAMP accumulation in hearts and cardiac myocytes. Since EGF does not increase cAMP accumulation in all tissues, we investigated the possibility that the expression of a specific isoform of adenylyl cyclase (AC) was necessary to observe EGF-elicited stimulation of cAMP accumulation. HEK 293 cells were transfected with different isoforms of AC, and the ability of EGF to increase AC activity as well as elevate cAMP accumulation was determined. In cells transfected with AC I, II, V, and VI cDNAs, neither the expression nor the amount of the two isoforms of Gs alpha (45 and 52 kDa) were altered. Similarly, EGF-elicited phosphorylation of cellular proteins on tyrosine residues in various transfectants was unaltered. However, EGF increased AC activity and elevated cAMP accumulation only in cells expressing the rat and canine ACV. EGF did not alter either AC activity or cAMP accumulation in cells overexpressing types I, II, and VI isozymes. As assessed by the ability of an anti-Gs alpha antibody to obliterate the effect, stimulation of AC activity in AC V transfectants involved the participation of Gs alpha, a finding consistent with previous data concerning EGF effects on cardiac AC (Nair, B. G., Parikh, B., Milligan, G., and Patel, T. B. (1990) J. Biol. Chem. 265, 21317-21322). Thus we conclude that the expression of AC V isoform confers specificity to the ability of EGF to stimulate AC activity.

Transforming growth factor-beta 1 modulates adenylyl cyclase signaling elements and epidermal growth factor signaling in cardiomyocytes

Studies presented in this report were designed to investigate the effects of transforming growth factor-beta 1 (TGF-beta 1) on epidermal growth factor (EGF)-mediated stimulation of cAMP accumulation in cardiac myocytes and elucidate the mechanism(s) involved in this modulation. TGF-beta 1 (20 pM) treatment of cardiac myocytes, in a time-dependent manner, decreased the ability of EGF (100 nM) to increase cAMP accumulation. Significant attenuation of EGF-elicited cAMP accumulation was observed 2 h after exposure to TGF-beta 1 and 18 h after addition of TGF-beta 1, the ability of EGF to increase cAMP accumulation was completely obliterated. TGF-beta 1 neither decreased immunoprecipitable EGF receptors in membranes from cardiomyocytes nor altered the specific binding of [125I]EGF to cardiomyocyte membranes. However, TGF-beta 1 decreased the ability of EGF to phosphorylate membrane proteins on tyrosine residues. TGF-beta 1 treatment of cardiomyocytes also decreased the ability of forskolin to augment cAMP accumulation in intact cells and stimulate adenylyl cyclase activity. Similarly, in membranes of TGF-beta 1-treated cells, neither isoproterenol nor EGF stimulated adenylyl cyclase activity. Interestingly, as assessed by the ability of A1F4- to stimulate adenylyl cyclase, TGF-beta 1 did not alter the coupling between Gs and catalytic subunits. Likewise, TGF-beta 1 did not alter the functional activity of the inhibitory regulatory element of the system, Gi. Western analysis of cellular proteins revealed that TGF-beta 1 did not alter the amounts of Ga alpha, Gi alpha 2, and Gi alpha 3. We conclude that TGF-beta 1 attenuates EGF-elicited cAMP accumulation in cardiomyocytes, in part, by decreasing the EGF receptor kinase function and that TGF-beta 1-mediated alterations in the activity of adenylyl cyclase catalytic subunit also contribute toward the regulation of adenylyl cyclase by various agonists.

Role of heterotrimeric G-proteins in epidermal growth factor signalling

Since in 1986 it was reported that a pertussis toxin-sensitive substrate was involved in the Ca2+ signal induced by epidermal growth factor (EGF) in rat hepatocytes, much evidence accumulated to implicate heterotrimeric G-proteins in EGF action. EGF can also induce a cyclic AMP signal, but while the generation of a Ca2+ signal appears to be quite general in EGF action, the increase in cyclic AMP occurs only in few cell types. In non-transformed cell types these effects appear to involve G-proteins. EGF not only induces cell proliferation but also interacts with hormones in the short-term control of cell function in quiescent cells. Most of the known interactions are on cyclic AMP mediated hormone effects, and in many cases, the interaction between EGF and hormones involves G-proteins. Here we review the evidence accumulated in recent years that implicate G-proteins in EGF action. An understanding of the mechanisms involved may reveal new mechanisms of G-protein regulation and will contribute to our knowledge of EGF function and signal transduction.

Usefulness of increased myocardial cyclic adenosine 3',5'-monophosphate content as a sign of rejection after cardiac transplantation

Cardiac allograft rejection represents a series of cellular and molecular events triggered by the recognition of the graft by the host immune system. One of the second messenger systems involved in mitogenic mechanisms is the cyclic adenosine 3',5'-monophosphate (cAMP)-coupled signaling system. The aim of this preliminary study was to evaluate whether rejection after cardiac transplantation is accompanied by changes in the expression of cAMP. Myocardial cAMP content was determined by radioimmunoassay in endomyocardial biopsy specimens taken during routine follow-up after cardiac transplantation with or without cellular and/or vascular (i.e., coronary vasculopathy) rejection, respectively. Analysis of the different subgroups of patients showed that patients without any signs of rejection (no vasculopathy, no cellular rejection) had the lowest myocardial cAMP content (1.41 +/- 0.12 pmol/mg wet weight). Patients with either cellular or vascular rejection had significantly higher myocardial cAMP levels (2.25 +/- 0.29 and 2.24 +/- 0.59 pmol/mg wet weight, respectively, p < 0.05). Patients with both cellular rejection and coronary vasculopathy had the highest cAMP levels (5.95 +/- 1.6 pmol/mg wet weight; p < 0.001). We speculate that cAMP may play a functional role in mediating rejection induced by mitogenic factors activated after cardiac transplantation, suggesting a possible "cross-talk" between different cellular signaling pathways.

Adrenomedullin stimulates two signal transduction pathways, cAMP accumulation and Ca2+ mobilization, in bovine aortic endothelial cells

The biological action of adrenomedullin, a novel hypotensive peptide, on bovine aortic endothelial cells, was examined. The specific binding of adrenomedullin to these cells was observed, and adrenomedullin was found to induce intracellular cAMP accumulation in a dose-dependent manner. EC50 for the cAMP accumulation was about 100 times lower than the apparent IC50 for the binding assay. Adrenomedullin also induced increase of intracellular free Ca2+ in endothelial cells in a dose-dependent manner. The Ca2+ response to adrenomedullin was biphasic with an initial transient increase due to the release from thapsigargin-sensitive intracellular Ca2+ storage and a prolonged increase by influx through the ion channel on the plasma membrane. This intracellular free Ca2+ increase resulted from phospholipase C activation and inositol 1,4,5-trisphosphate formation, and seemed to cause nitric oxide synthase activation by monitoring intracellular cGMP accumulation. Both cAMP accumulation and Ca2+ increased responses to adrenomedullin were mediated by cholera toxin-sensitive G protein, but the two signal transduction pathways were independent. Thus, the results suggest that adrenomedullin elicits the hypotensive effect through at least two mechanisms, a direct action on vascular smooth muscle cells to increase intracellular cAMP and an action on endothelial cells to stimulate nitric oxide release, with both leading to vascular relaxation.

Regulation of cardiac adenylyl cyclase by epidermal growth factor (EGF). Role of EGF receptor protein tyrosine kinase activity

We have shown previously that the alpha subunit of the stimulatory GTP binding regulatory component of adenylyl cyclase (Gs alpha) mediates epidermal growth factor (EGF)-elicited stimulation of rat cardiac adenylyl cyclase (Nair et al., J Biol Chem 265: 21317-21322, 1990). Employing purified protein phosphotyrosine phosphatase, and benzylidene derivatives (tyrphostins: compounds 11 and 12) that selectively inhibit EGF receptor protein tyrosine kinase (EGFRK) activity, the role of EGFRK in EGF-mediated stimulation of cardiac adenylyl cyclase was investigated. The ability of the tyrphostins to inhibit the EGFRK activity in cardiac membranes was determined by monitoring tyrosine phosphorylation of either the 170 kDa protein or immunoprecipitated EGF receptor at 0 degrees and room temperature, respectively. Compounds 11 and 12, in a concentration-dependent manner, inhibited EGF receptor tyrosine kinase activity. In assays of adenylyl cyclase activity neither compound 11 nor compound 12 altered Gpp(NH)p- or isoproterenol-stimulated activity. However, both compounds, in a concentration-dependent manner, attenuated the ability of EGF to stimulate adenylyl cyclase activity without altering specific binding of [125I]EGF to cardiac membranes. Similarly, protein phosphotyrosine phosphatase obliterated the ability of EGF, but not isoproterenol, to stimulate adenylyl cyclase. Thus, we conclude that protein tyrosine kinase activity of the EGF receptor is essential for the stimulation of cardiac adenylyl cyclase by EGF.

Epidermal growth factor produces inotropic and chronotropic effects in rat hearts by increasing cyclic AMP accumulation

Previously we have shown that epidermal growth factor (EGF) stimulates cardiac adenylyl cyclase and increases cAMP accumulation in the rat heart (Nair et al., Biochem. J. 264, 563-571, 1989). Moreover, we have shown that the stimulation of adenylyl cyclase by EGF in heart is mediated via activation of the stimulatory GTP binding regulatory protein Gs alpha (Nair et al., J. Biol. Chem. 265, 21317-21322, 1990). Since cAMP increases the beating rate of hearts, studies were performed to investigate the effects of EGF on mechanical function of the heart and the role of cAMP in mediating the cardiac effects of EGF. In isolated perfused rat hearts EGF (15 nM) decreased perfusion pressure, increased ventricular contractility and heart rate in a manner similar to that observed with the beta-adrenergic receptor agonist isoproterenol (10 nM). In the presence of the adenosine A1 receptor agonist (-)-N6-(R-phenylisopropyl)-adenosine (PIA, 100 nM) which via activation of the inhibitory GTP binding protein Gi inhibits adenylyl cyclase, the effects of EGF on cAMP accumulation in the heart were markedly attenuated. PIA also decreased the ability of EGF and isoproterenol to alter cardiac contractility and beating rate. However, PIA did not attenuate the increase in heart rate and contractility induced by the alpha-adrenergic agonist phenylephrine which does not stimulate cAMP accumulation in the heart. These data suggest that EGF alters cardiac function by increasing cellular cAMP accumulation.