Different pathways of inositol phosphate metabolism in intact neonatal rat hearts and isolated cardiomyocytes

Developmental aspects of cardiac Ca(2+) signaling: interplay between RyR- and IP(3)R-gated Ca(2+) stores

The dominant mode of intracellular Ca(2+) release in adult mammalian heart is gated by ryanodine receptors (RyRs), but it is less clear whether inositol 1,4,5-trisphosphate (IP(3))-gated Ca(2+) release channels (IP(3)Rs), which are important during embryogenesis, play a significant role during early postnatal development. To address this question, we measured confocal two-dimensional Ca(2+) dependent fluorescence images in acutely isolated neonatal (days 1 to 2) and juvenile (days 8-10) rat cardiomyocytes, either voltage-clamped or permeabilized, where rapid exchange of solution could be used to selectively activate the two types of Ca(2+) release channel. Targeting RyRs with caffeine produced large and rapid Ca(2+) signals throughout the cells. Application of ATP and endothelin-1 to voltage-clamped, or IP(3) to permeabilized, cells produced smaller and slower Ca(2+) signals that were most prominent in subsarcolemmal regions and were suppressed by either the IP(3)R-blocker 2-aminoethoxydiphenylborate or replacement of the biologically active form of IP(3) with its L-stereoisomer. Such IP(3)R-gated Ca(2+) releases were amplified by Ca(2+)-induced Ca(2+) release (CICR) via RyRs since they were also reduced by compounds that block the RyRs (tetracaine) or deplete the Ca(2+) pools they gate (caffeine, ryanodine). Spatial analysis revealed both subsarcolemmal and perinuclear origins for the IP(3)-mediated Ca(2+) release events RyR- and IP(3)R-gated Ca(2+) signals had larger magnitudes in juvenile than in neonatal cardiomyocytes. Ca(2+) signaling was generally quite similar in atrial and ventricular cardiomyocytes but showed divergent development of IP(3)-mediated regulation in juveniles. Our data suggest that an intermediate stage of Ca(2+) signaling may be present in developing cardiomyocytes, where, in addition to RyR-gated Ca(2+) pools, IP(3)-gated Ca(2+) release is sufficiently large in magnitude and duration to trigger or contribute to activation of CICR and cardiac contraction.

Inositol phospholipids localized to caveolae in rat heart are regulated by alpha1-adrenergic receptors and by ischemia-reperfusion

Postischemic reperfusion of rat or mouse hearts causes generation of inositol (1,4,5)trisphosphate [Ins(1,4,5)P3] and the initiation of arrhythmias. In the current study we investigated the possibility that the enhanced Ins(1,4,5)P3 generation in postischemic reperfusion was associated with an increased availability of the precursor lipid phosphatidylinositol(4,5)bisphosphate (PIP2) for alpha1-adrenergic receptor-activated phospholipase C (PLC). Isolated-perfused rat hearts were labeled with [3H]inositol and subjected to ischemia-reperfusion or stimulation with norepinephrine under normoxic conditions. Caveolar fractions were prepared by buoyant density sucrose gradient centrifugation. [3H]PIP2 was concentrated in caveolae, along with Galphaq and PLCbeta1b. Caveolae contained only 27.3 +/- 6.9% (means +/- SE, n = 6) of the total alpha1-adrenergic receptor complement of the heart. These did not migrate to PIP2-containing caveolar fractions with norepinephrine stimulation under normoxic conditions, even though caveolar PIP2 was depleted. In contrast, [3H]PIP2 in caveolae increased during 2 min of reperfusion, independently of norepinephrine release and thus of alpha1-adrenergic receptor activation. The increased PIP2 in the caveolar fractions where signaling proteins are concentrated may be critical for the heightened generation of Ins(1,4,5)P3 in early reperfusion.

Ca2+-dependent elevation of inositol 1,4,5-trisphosphate level induced by freezing or homogenization of tissues and cells

Various cells and tissues contain high basal levels of inositol 1,4,5-trisphosphate, raising questions about the functional significance of inositol 1,4,5-trisphosphate in some tissues such as the heart. We used intact tissue and isolated cells from heart and liver of adult rats to examine if different fixation procedures might artificially elevate the level of inositol 1,4,5-trisphosphate. The basal level of inositol 1,4,5-trisphosphate in intact, freeze-clamped cardiac tissue from adult rats was 10 times higher than in isolated, non-frozen cardiomyocytes, while freeze-clamped liver contained approximately 4 times higher inositol 1,4,5-trisphosphate levels than isolated, non-frozen hepatocytes. Stimulation with norepinephrine induced a significant increase in the inositol 1,4,5-trisphosphate level in isolated cardiomyocytes, whereas no significant increase was observed in freeze-clamped cardiac tissue. Freezing of isolated cardiomyocytes or hepatocytes before extraction increased basal inositol 1,4,5-trisphosphate levels 3 times. In cellular homogenates prepared in the presence of EGTA and stored at 4 degrees , readdition of calcium resulted in a time-dependent increase in inositol 1,4,5-trisphosphate mass and a decrease in the mass of phosphatidylinositol 4,5-bisphosphate (PIP(2)). The reaction was essentially complete within 30 sec. in homogenates from cardiomyocytes, while PIP(2) hydrolysis was slower in hepatocyte homogenates. Perfusion of intact rat hearts with EGTA present during the last 2 min. of perfusion, followed by freeze-clamping, resulted in basal inositol 1,4,5-trisphosphate levels comparable to those in isolated cardiomyocytes, and norepinephrine stimulation increased inositol 1,4,5-trisphosphate mass by approximately 80%. The presence of EGTA did not significantly affect PIP(2) levels in perfused hearts. The results suggest that freezing or homogenization of intact tissue and isolated cells may result in Ca(2+)-dependent activation of phospholipase C, leading to high basal inositol 1,4,5-trisphosphate levels that may mask agonist-induced changes.

Positive inotropic effect of ouabain on isolated heart is accompanied by activation of signal pathways that link Na+/K+-ATPase to ERK1/2

Exposure of cultured rat cardiac myocytes to ouabain is known to cause the interaction of Na+/K+-ATPase with adjacent proteins, leading to activation of multiple signal transduction pathways, regulation of growth-related genes, and hypertrophy. The aim of this work was to determine if the proximal signaling events identified in cultured myocytes also occur in isolated intact hearts of rat and guinea pig in response to positive inotropic doses of ouabain. Langendorff rat heart preparations were exposed to 50 microM ouabain to produce positive inotropy without toxicity, and assayed for Src kinase, protein kinase C, and extracellular signal-regulated kinases 1 and 2 (ERK(1/2)). These kinases were rapidly activated by ouabain as in cultured cells. In isolated guinea pig hearts, 1 microM ouabain caused ERK(1/2) activation comparable to the effect of 50 microM ouabain in rat heart and consistent with the higher ouabain sensitivity of the contractility of guinea pig heart. These data show that the proximal ouabain-induced signal pathways previously noted in cultured cells are not artifacts of dispersion/culturing of myocytes, and are not the peculiar properties of the rat heart with its relatively low ouabain sensitivity. They also suggest that treatment with positive inotropic doses of cardiac glycosides is likely to be associated with changes in the cardiac phenotype.

Interaction of chelerythrine with inositol phosphate metabolism

Chelerythrine, a potent inhibitor of protein kinase C (PKC), was evaluated for its effect on inositol phosphate (IP) metabolism in newborn rat cardiomyocytes in culture. In a first step, we evaluated the effect of chelerythrine on IP accumulation in basal conditions. For a 10(-4) M dose, 5-phosphatase activity (which dephosphorylates IP3 into IP2) was completely blocked and we observed a large increase in IP accumulation limited to IP2 without any increase in IP3, strongly suggesting that chelerythrine at this dose modifies IP metabolism. At a lower dose (10(-5) M) of chelerythrine, which did not modify IP accumulation and 5-phosphatase activity in basal conditions, the response to angiotensin II stimulation was completely abolished by the addition of chelerythrine. We conclude thus that chelerythrine, even at 10(-5) M, interacts markedly with IP metabolism, and caution should be exerted when interpreting the results obtained with this drug, which is still currently used at this dose.

Role for PKC in the adenosine-induced decrease in shortening velocity of rat ventricular myocytes

We previously demonstrated that both adenosine receptor activation and direct activation of protein kinase C (PKC) decrease unloaded shortening velocity (V(max)) of rat ventricular myocytes. The goal of this study was to further investigate a possible link among adenosine receptors, phosphoinositide-PKC signaling, and V(max) in rat ventricular myocytes. We determined that the adenosine receptor agonist R-phenylisopropyladenosine (R-PIA, 100 microM) and the alpha-adrenergic receptor agonist phenylephrine (Phe, 10 microM) increased turnover of inositol phosphates. PKC translocation from the cytosol to the sarcolemma was used as an indicator of PKC activation. Western blot analysis demonstrated an increased PKC-epsilon translocation after exposure to R-PIA, Phe, and the PKC activators dioctanoylglycerol (50 microM) and phorbol myristate acetate (1 microM). PKC-alpha, PKC-delta, and PKC-zeta did not translocate to the membrane after R-PIA exposure. Finally, PKC inhibitors blocked R-PIA-induced decreases in V(max) as well as Ca(2+)-dependent actomyosin ATPase in rat ventricular myocytes. These results support the conclusions that adenosine receptors activate phosphoinositide-PKC signaling and that adenosine receptor-induced PKC activation mediates a decrease in V(max) in ventricular myocytes.

Ca(2+)-activated but not G protein-mediated inositol phosphate responses in rat neonatal cardiomyocytes involve inositol 1,4, 5-trisphosphate generation

Inositol phosphate (InsP) responses to receptor activation are assumed to involve phospholipase C cleavage of phosphatidylinositol 4,5-bisphosphate to generate Ins(1,4,5)P(3). However, in [(3)H]inositol-labeled rat neonatal cardiomyocytes (NCM) both initial and sustained [(3)H]InsP responses to alpha(1)-adrenergic receptor stimulation with norepinephrine (100 microM) were insensitive to the phosphatidylinositol 4,5-bisphosphate-binding agent neomycin (5 mM). Introduction of 300 microM unlabeled Ins(1,4, 5)P(3) into guanosine 5'-3-O-(thio)triphosphate (GTPgammaS)-stimulated, permeabilized [(3)H]inositol-labeled NCM increased [(3)H]Ins(1,4,5)P(3) slightly but did not significantly reduce levels of its metabolites [(3)H]Ins(1,4)P(2) and [(3)H]Ins(4)P, suggesting that these [(3)H]InsPs are not formed principally from [(3)H]Ins(1,4,5)P(3). In contrast, the calcium ionophore A23187 (10 microM) provoked [(3)H]InsP responses in intact NCM which were sensitive to neomycin, and elevation of free calcium in permeabilized NCM led to [(3)H]InsP responses characterized by marked increases in [(3)H]Ins(1,4,5)P(3) (2.9 +/- 0.2% of total [(3)H]InsPs after 20 min of high Ca(2+) treatment in comparison to 0. 21 +/- 0.05% of total [(3)H]InsPs accumulated after 20 min of GTPgammaS stimulation). These data provide evidence that Ins(1,4, 5)P(3) generation is not a major contributor to G protein-coupled InsP responses in NCM, but that substantial Ins(1,4,5)P(3) generation occurs under conditions of Ca(2+) overload. Thus in NCM, Ca(2+)-induced Ins(1,4,5)P(3) generation has the potential to worsen Ca(2+) overload and thereby aggravate Ca(2+)-induced electrophysiological perturbations.

Effect of concomitant beta-adrenoceptor stimulation on alpha 1-adrenoceptor-mediated increase of inositol-1,4,5-trisphosphate mass in adult rat cardiomyocytes

The aim of the present study was to investigate the accumulation of inositol-1,4,5-trisphosphate (IP3) in isolated adult rat ventricular cardiomyocytes after alpha 1- and beta-adrenoceptor stimulation, separate and in combination, in order to elucidate a possible influence of concomitant beta-adrenoceptor stimulation on the alpha 1-adrenoceptor stimulated response. IP3 was measured by a radioligand binding assay based on an (1,4,5)IP3-specific binding protein from bovine adrenal cortex. The basal IP3 content was 4.06 +/- 0.31 pmol/mg protein (N = 56). alpha 1-Adrenoceptor stimulation resulted in a rapid increase in the IP3 level, which reached a plateau, 50-80% above basal level, at 10-30 sec. The plateau lasted at least up to 120 sec., while at 300 sec. there was no significant difference between control values and values after alpha 1-adrenoceptor stimulation. Li+ did not affect either the basal IP3 level, or the magnitude or time course of alpha 1-adrenoceptor-stimulated IP3 accumulation. Combined adrenoceptor stimulation gave a similar response as separate alpha 1-adrenoceptor stimulation, whereas there was no significant change in the IP3 level after beta-adrenoceptor stimulation. No inhibitory influence of simultaneous beta-adrenoceptor stimulation on the alpha 1-adrenoceptor-stimulated increase of IP3 mass was revealed.

Effects of platelet-activating factor on intracellular Ca2+ concentration and contractility in isolated cardiomyocytes

We investigated the effects of platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) on intracellular Ca2+ concentration ([Ca2+]i) and cell length in isolated and field-stimulated rat cardiomyocytes. [Ca2+]i and cell length of field-stimulated cells were determined simultaneously by confocal laser scan microscopy by using the fluorescent Ca2+ dye Fluo-3. PAF (10(-12)-10(-8) M) inhibited systolic [Ca2+]i increase in a time- and concentration-dependent manner. Maximal effects were observed after an incubation time of 6-8 min, resulting in a 17% (10(-12) M), 41% (10(-10) M), and 52% (10(-8) M PAF) inhibition of systolic [Ca2+]i increase. A time- and concentration-dependent decrease in simultaneously measured cell shortening also was demonstrated. Cell shortening was inhibited by 10% (10(-12) M), 32% (10(-10) M), and 50% (10(-8) M) after an incubation time of 8 min. The effects of PAF could be antagonized by the PAF-receptor antagonist WEB 2170. These data demonstrate that PAF receptor-dependently induces a negative inotropic effect, which is correlated with a decrease in systolic [Ca2+]i and is most likely not due to a decrease in myofilament sensitivity.

Analysis of inositol phosphates in heart tissue using anion-exchange high-performance liquid chromatography

The pathways of release and metabolism of inositol phosphates in intact heart tissue are different from those observed in isolated cardiomyocytes in culture. Thus, it is essential that methods are available for the quantitation of inositol phosphates in intact tissue preparations. This manuscript describes methods which allow the quantitation of inositol phosphates in different heart preparations including isolated atria and intact perfused heart. The availability of such methods should facilitate study of the role of inositol phosphates in cardiac control mechanisms under physiological and pathological conditions.

Load-induced growth responses in isolated adult rat hearts. Role of the AT1 receptor

BACKGROUND

Stimulation of the angiotensin II type 1 (AT1) receptor by angiotensin II appears to be mandatory for the acute load-induced hypertrophic response of cultured neonatal rat cardiocytes, but its role in the adult heart is controversial. We tested the hypothesis that AT1 receptor blockade will inhibit the acute induction of proto-oncogenes and protein synthesis by the elevation of systolic wall stress in isolated beating adult rat hearts.

METHODS AND RESULTS

Using the established isovolumic perfused heart preparation under constant coronary flow, we found that an increment in left ventricular balloon volume generated an increase in systolic wall stress. The induction of left ventricular c-fos and c-myc mRNA (Northern blotting) was assessed in hearts subjected to increased systolic load without AT1 blockade (No AT1, n = 11) and with AT1 blockade (AT1, n = 11, losartan 40 mg.kg-1.d-1 x 5 days followed by 10(-5) mol/L infusion during perfusion). Flaccid hearts (no left ventricular balloon) served as controls (C, n = 9). The stimulation of new protein synthesis in response to increased systolic load was measured by incorporation of [3H]phenylalanine into cardiac proteins. Elevation of systolic load was associated with a twofold (P < .05) increase in c-fos and c-myc mRNA levels that was not blocked by losartan. The rate of [3H]phenylalanine incorporation into cardiac proteins was increased 2.7-fold (P < .01) in hearts subjected to increased systolic load compared with control hearts. However, AT1 receptor blockade with losartan did not prevent the stimulation of [3H]phenylalanine incorporation (881 +/- 97 versus 923 +/- 82 nmol.g protein-1.h-1, P = NS).

CONCLUSIONS

In contrast with immature myocytes subjected to stretch, the acute growth responses induced by systolic pressure overload in adult rat hearts do not depend on AT1 receptor activation.

Reduction in Gh protein expression is associated with cytodifferentiation of vascular smooth muscle cells

Gh, a high molecular weight GTP-binding protein that couples alpha 1-adrenoceptors in heart and liver to phosphatidylinositol (PtdIns)-specific phospholipase C (PLC), has recently been shown to be a tissue transglutaminase type II. Transglutaminases have been suggested to play a role in the maintenance of blood vessel structure, and therefore it is possible that changes in their expression may accompany pathological states which involve phenotypic modulation of smooth muscle. Hence, we investigated the expression of Gh during differentiation of rat aortic smooth muscle cells in culture. Gh content was reduced markedly in cultured smooth muscle cells compared to freshly isolated cells as determined by Western blotting using a Gh-specific monoclonal antibody. In contrast, the level of Gq, a heterotrimeric G-protein that couples alpha 1-adrenoceptors to PLC, was maintained throughout the culture period. These findings indicate that changes in Gh expression accompany phenotypic modulation of vascular smooth muscle cells. These changes in Gh protein expression may be important in the altered responsiveness of vessels in pathological disease states.

Phosphoinositide-generated messengers in cardiac signal transduction

A multitude of agonists like e.g. endothelin-1, angiotensin-II, serotonin, thrombin, histamine and vasopressin as well as alpha 1-adrenergic and muscarinic stimulation lead to stimulation of the phosphoinositide cycle in the heart. Besides the seven membrane spanning-domain receptor-coupled stimulation of the key enzyme of the phosphoinositide cycle, phospholipase C-beta, another class of hormones, growth factors, also couple to the phosphoinositide cycle, now through receptors with intrinsic tyrosine kinase activity that can phosphorylate and stimulate the phospholipase C-gamma isozyme. In this review we summarize the multitude of receptor (sub)types, G-protein-subunit- and phospholipase C-isozymes that are present in the heart. Furthermore, generation of second messengers and cellular responses are described together with the (patho)physiological implications for the heart of phosphoinositide cycle activation and second messenger accumulation.

Myocardial contractile response and IP3, cAMP and cGMP interrelationships

An experimental study in the perfused working normal and pressure overloaded rat heart. A mini review based on a doctoral thesis.

The norepinephrine-stimulated inositol phosphate response in human atria

Inositol phosphate release and metabolism were studied in right atrial appendages obtained from 18 patients undergoing coronary artery bypass surgery and/or mitral valve replacement. [3H]Inositol-labeled human atria contained inositol(1,4. 5)trisphosphate, inositol(1,4)bisphosphate and the 1- (or 3) and 4-isomers of inositol monophosphate. Addition of norepinephrine (100 mumol/l) activated the release of inositol phosphates, as indicated by increased [3H]inositol label in all of these inositol phosphates. However, the phosphorylation product of inositol (1.4.5)trisphosphate, inositol-(1,3,4,5)tetrakisphosphate, and its metabolic products were not detected, either in control or stimulated atria. Similar inositol phosphate profiles were observed in rat right atria. Furthermore, both human and rat atria contained high concentrations of inositol(1,4,5)trisphosphate, which were not observed to increase with norepinephrine stimulation. The inositol phosphate responses to norepinephrine in rat and human cardiac tissue appear to be similar, except for the generally lower activity observed in human tissue. Thus, the rat provides a suitable model for the study of cardiac phosphatidylinositol turnover.

Isolation of neonatal cardiomyocytes reduces the expression of the GTP-binding protein, Gh

alpha 1-Adrenoceptors in most tissues couple with the heterotrimeric GTP-binding protein Gq, the alpha subunit of which activates the beta-isoforms of phospholipase C. However, in heart (and in liver) alpha 1-adrenoceptors have been reported to couple to a high molecular weight GTP-binding protein. Gh, which functions both as a type II transglutaminase and as a receptor coupling protein. Gh activates a phospholipase isoform distinct from phospholipase C-beta. Here we report that isolation and culture of neonatal cardiomyocytes decreased the expression of Gh without reducing the content of Gq or Gi. Gh was readily detected in extracts from intact neonatal and adult heart tissues. The expression of Gh thus appears to be a feature of intact cardiac tissue.

Inositol phosphates in the heart: controversy and consensus

Numerous studies have addressed various aspects of inositol phosphate release and metabolism in myocardial preparations, and many different viewpoints have been expressed. The various results and interpretations presented often appear confusing and extracting a consensus view can be difficult. The differences often derive from the differing cardiac preparations used, especially isolated cells versus intact tissue. Despite these problems there are aspects where consensus prevails. Both the metabolism and the functional activity of inositol phosphates in heart appear to differ from those previously described in non-excitable cells. Inositol phosphates do not appear to be of major importance in the control of cardiac function under physiological conditions but may well have greater influence under pathological conditions such as myocardial ischaemia and reperfusion. Hopefully, the near future will see remaining controversies resolved.

Angiotensin II-induced growth responses in isolated adult rat hearts. Evidence for load-independent induction of cardiac protein synthesis by angiotensin II

Cardiac myocyte hypertrophy often occurs in response to both hemodynamic and neurohumoral factors. To study whether activation of the renin-angiotensin system by itself may induce a cardiac growth response, the acute effects of angiotensin II on cardiac protein synthesis were studied in isolated rat hearts. New protein synthesis in isolated buffer-perfused adult rat hearts was measured by incorporation of [3H]phenylalanine into cardiac proteins during a 3-hour perfusion protocol. Angiotensin II (1 x 10(-8) mol/L), administered alone or in combination with the alpha 1-blocker prazosin (1 x 10(-7) mol/L), stimulated protein synthesis in both ventricles. The rate of [3H]phenylalanine incorporation into cardiac proteins was 3.9-fold (P < .005) and 2.6-fold (P < .01) higher in angiotensin II-perfused (n = 6) than in vehicle-perfused (n = 6) left and right ventricles, respectively. The induction of new protein synthesis by angiotensin II was blocked by the angiotensin II type 1 (AT1) receptor antagonist losartan (1 x 10(-7) mol/L, n = 5). To study the pathways of angiotensin signal transduction, protein kinase C (PKC)-epsilon as well as cardiac c-fos and c-jun mRNA levels were analyzed. Angiotensin II (1 x 10(-8) mol/L, n = 20) resulted in a transient translocation of PKC-epsilon from the cytosol to the cellular membrane. However, compared with phorbol ester stimulation (phorbol 12-myristate 13-acetate [PMA], 1 x 10(-7) mol/L; n = 20), angiotensin II effects on PKC translocation were significantly less pronounced and required a more prolonged stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial inositoltrisphosphate is depressed by dibutyryl cAMP. An experimental study in the isolated working rat heart

A possible interrelation between IP3 and cAMP was studied in rat myocardium through circumvention of the receptor mediated stimulatory step of adenylyl cyclase by the administration of dibutyryl cAMP (db-cAMP). Changes in IP3 and cyclic nucleotide contents were correlated to changes in contractility after 40 min of beta- and alpha-adrenergic stimulation. Rat hearts (n = 23) were perfused with Krebs-Henseleit buffer in a modified Langendorff apparatus as a working preparation. The hearts were allocated to perfusion as control (n = 6); or with phenylephrine (10(-6) mol L-1, n = 6); (-)-isoproterenol (10(-6) mol L-1, n = 6); db-cAMP (2 x 10(-4) mol L-1, n = 5). All hearts were freeze-clamped after 40 min of perfusion. Phenylephrine produced a slow increase in maxdP/dt reaching a maximal value after 10 min (P < 0.05); thereafter it decreased, reaching the control level at 30 min. Isoproterenol perfusion resulted in an early (20 s) increase in maxdP/dt (P < 0.05). Over the next 10s maxdP/dt decreased markedly reaching an inflection point at 30 s. Thereafter only a slow increase during the rest of the perfusion was seen. Dibutyryl cAMP increased maxdP/dt slowly during the whole perfusion period reaching maximum after 40 min. Cyclic-AMP was increased by 21% after 40 min of phenylephrine perfusion while the corresponding increases by isoproterenol and db-cAMP were 131 and 105%, respectively (P < 0.05). Phenylephrine increased IP3 content to the same extent as isoproterenol perfusion (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol phosphate release and metabolism in rat left atria

The phosphatidylinositol (PtdIns) turnover pathway in intact heart tissue differs from that in most cell types in that products of the inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] kinase pathway are not detected in 3H-labeling studies. In contrast, Ins(1,4,5)P3 kinase products are detected in isolated neonatal cardiomyocytes. To understand the basis for the observed properties of the cardiac pathway, a detailed study of inositol phosphate (InsP) release has been undertaken by using isolated adult rat left atria. Addition of norepinephrine to 3H-labeled atria caused a slow increase in 3H-labeled Ins(1,4,5)P3 and a more rapid increase in 3H-labeled Ins(1,4)P2, its immediate dephosphorylation product. The mass of Ins(1,4,5)P3 was high in unstimulated atria (13.5 +/- 1.1 pmol/mg tissue, mean +/- SEM, n = 4) and did not change with stimulation. Measurements of the specific activities of Ins(1,4,5)P3 and PtdIns(4,5)P2 provided an estimate of the turnover rate of Ins(1,4,5)P3 that was 20- to 40-fold lower than the rate of accumulation of 3H label in InsP1 and InsP2. In agreement with this, specific activities of InsP1 and InsP2 were higher than the specific activity of InsP3 in both control and stimulated atria. Neomycin (5 mmol/L) did not inhibit the accumulation of 3H-labeled InsP1 and InsP2 in left atria, even though it reduced the accumulation of 3H label in Ins(1,4,5)P3, providing evidence that InsP1 and InsP2 do not derive primarily from Ins(1,4,5)P3. Stimulation with norepinephrine for 20 minutes resulted in a parallel decrease in 3H-labeled Ins(1,4,5)P3 and in Ins(1,4,5)P3 mass, demonstrating that atria do not contain two different pools of Ins(1,4,5)P3.(ABSTRACT TRUNCATED AT 250 WORDS)

Ins 1,4,5-P3 and Ca2+ signaling in quiescent neonatal cardiac myocytes

Activation of alpha 1-adrenergic receptors in neonatal cardiac myocytes results in changes in contractile activity and the induction of hypertrophic growth. The biochemical mechanisms responsible for these diverse effects are not yet established, but presumably involve the associated alpha 1-adrenergic stimulation of phosphatidylinositol (PI) hydrolysis, with concomitant generation of Ins 1,4,5-P3 and diacylglycerol. This study examined whether alpha 1-adrenergic generation of Ins 1,4,5-P3 in intact, quiescent, neonatal cardiac myocytes resulted in a Ca2+ signal. Stimulation of myocytes with norepinephrine in the presence of propranolol caused accumulation of inositol mono-, bis and trisphosphates. However, alpha 1-adrenergic stimulation did not alter cytosolic free Ca2+ levels in 85% of the myocytes examined. Direct generation of Ins 1,4,5-P3, by photolysis of microinjected caged Ins 1,4,5-P3, was also unable to alter cytosolic free Ca2+ levels, despite the presence of Ins 1,4,5-P3 receptors. Taken together, these data indicated that alpha 1-adrenergic stimulation did not initiate Ca2+ signaling because Ins 1,4,5-P3-induced Ca2+ mobilization was not operative in quiescent neonatal cardiac myocytes. Normal excitation-contraction Ca2+ handling mechanisms were present in these cells, as illustrated by depolarization- and caffeine-induced Ca2+ transients. Analysis of these same myocytes following 48 h in the presence of norepinephrine and propranolol showed a 40% increase in the ratio of protein to DNA and a 350% increase in release of atrial naturietic factor, compared to control cells, indicating the normal operation of alpha 1-adrenergic-induced hypertrophic growth. Therefore, the assumption that Ca(2+)-dependent processes will be activated by receptor signaling pathways coupled to enhanced phosphatidylinositol turnover in cardiac cells must be avoided. In addition, the data presented in this study clearly indicated that an increase in cytosolic free Ca2+ was not necessary for the induction of alpha 1-adrenergic-mediated cardiac hypertrophy.

A low-affinity, low-molecular-mass endothelin-A receptor in neonatal rat heart

Endothelin receptors with endothelin-A (ETa) specificity were present in neonatal rat ventricle. However, in both receptor-binding studies and studies of inositol phosphate accumulation, these receptors had lower affinity for endothelin-1 than ETa receptors on isolated neonatal cardiomyocytes or adult left atria. Receptors in the three myocardial preparations were cross-linked to 125I-endothelin-1 and their molecular masses measured using SDS/PAGE. Receptors on left atria and neonatal cardiomyocytes had the expected molecular mass of 48 kDa, whereas the receptors in neonatal ventricle were smaller (38 kDa). Despite this, neonatal ventricles contained ETa receptor mRNA which was not different in size from that in the isolated cells (4.5 kb). Thus the 38 kDa ETa receptor present in neonatal ventricle appears to be transcribed from full-length ETa receptor mRNA and is possibly formed by processing of the 48 kDa receptor.

Inositol 1,4,5-trisphosphate receptor function in neonatal cardiomyocytes

We have previously reported that the metabolism of inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) is altered when rat neonatal ventricular cardiomyocytes are isolated and cultured. In the current study we show that the mass content of Ins(1,4,5)P3 is lower in the isolated cells than in the intact tissue. However, the properties of the Ins(1,4,5)P3 receptors were not different in the two preparations and the isolated cells remained insensitive to Ins(1,4,5)P3 in terms of 45Ca2+ release. Thus, despite the altered pattern of metabolism of Ins(1,4,5)P3 in isolated neonatal cells, the properties of the receptors were similar to those reported in other myocardial preparations.

Characterization of specific binding sites for alpha-trinositol (D-myo-inositol 1,2,6-trisphosphate) in rat tissues

Alfa-trinositol (or D-myo-inositol 1,2,6-trisphosphate) was recently found to, e.g., inhibit agonist-induced vasoconstriction and display antiinflammatory properties. However, its mechanism of action is unknown, although effects on Ca2+ fluxes, perhaps by interfering with endogenous inositol phosphate(s), have been suggested. Here we describe the existence of specific [3H]alpha-trinositol binding sites and compare these with binding sites for naturally occurring inositol phosphates. For this purpose we developed a tritiated analog of alpha-trinositol and used it in a centrifugation binding assay on extensively washed membranes from rat tissues. The degree of specific [3H] alpha-trinositol binding was markedly increased as a result of the many wash steps, indicating the existence of endogenous binding inhibitor(s). A single population of [3H] alpha-trinositol binding sites, displaying a KD of 159 nM and a Bmax of 71 pmol/mg protein, was present in cardiac membranes assayed at pH 7.4. Similar binding site densities were detected also in liver > lung > brain. The relative density of [3H] alpha-trinositol sites in cardiac membranes was 8-fold higher than [3H]Ins(1,4,5)P3 but 2-fold and 4-fold lower than [3H]Ins(1,3,4,5)P4 and [3H]InsP6 binding sites, respectively. Competition binding studies indicated the ability of Ins(1,3,4,5)P4 and InsP6, but not Ins(1,4,5)P3, to potently displace [3H] alpha-trinositol binding. Conversely, unlabelled alpha-trinositol showed relatively low potency vs. [3H]InsP6, but the novel inositol phosphate was virtually equipotent with Ins(1,3,4,5)P4 in inhibiting [3H]Ins(1,3,4,5)P4 binding. Finally, analyses of binding at different pH and ionic conditions revealed differences between alpha-trinositol and the three other previously studied inositol phosphates, although distinct similarities between alpha-trinositol and Ins(1,3,4,5)P4 were again observed.

Inositol-1,4,5-trisphosphate [ins(1,4,5)P3] and ins(1,4,5)P3 receptor concentrations in heart tissues

1. The isolation and culture of neonatal cardiomyocytes causes changes in the metabolism of inositol(1,4,5) trisphosphate (Ins(1,4,5)P3) from primarily dephosphorylation in the intact tissue to a combination of phosphorylation and dephosphorylation in the cultured cells (Woodcock et al. 1992). 2. The content of Ins(1,4,5)P3 was found to be higher in intact heart tissue than in the isolated neonatal cells (10.9 +/- 1.3 and 0.5 +/- 0.1 pmol/mg tissue, mean +/- s.e.m., n = 4, P < 0.002, respectively). 3. Despite this difference, Ins(1,4,5)P3 receptors in intact tissue and in isolated cells were not different in terms of affinity (8.0 +/- 1.7 and 10.9 +/- 1.6 nmol/L, n = 3, respectively) or concentration (143.3 +/- 20.5 and 91.2 +/- 16.0 fmol/mg protein, n = 3, respectively). 4. Thus, while there appears to be a relationship between the tissue content of Ins(1,4,5)P3 and its metabolism, no relationship to the properties of Ins(1,4,5)P3 receptors could be demonstrated.

Dynamic changes of myocardial inositoltrisphosphate and cyclic nucleotides: relationship to contractile response in the perfused working rat heart after adrenergic and muscarinic agonist stimulation

Initial and late effects by adrenergic and muscarinic agonists on inositol trisphosphate (IP3) and cyclic nucleotide levels were determined and correlated to mechanical response in perfused rat hearts. Forty-three rat hearts were perfused with Krebs-Henseleit buffer in a modified Langendorff apparatus as a working preparation. The hearts were perfused as controls (n = 11), or with noradrenaline (10(-6) mol l-1) (n = 21), or with carbachol (3 x 10(-7) mol l-1) (n = 11) added to the perfusion buffer. The hearts were frozen at 20 s, 30 s and 40 min after addition of noradrenaline and at 20 s and 40 min after addition of carbachol, and after 5 and 45 min of control perfusion. cAMP and cGMP were determined by radioligand methods and IP3 by a combined fast performance liquid chromatography (FPLC)-isotachophoretic method. cAMP increased by 36% within 20 s followed by a decrease (22%) during the 10 s following noradrenaline addition. After 40 min cAMP regained its value near that of 20 s. Noradrenaline perfusion did not influence IP3 levels during the first 30 s although the value at 40 min was significantly higher (59%). IP3 increased (42%) after 20 s of carbachol perfusion followed by a 25% decrease at 40 min. Sustained stimulation of beta-receptors (after 40 min in our model) resulted in a repeated increase in cAMP only, without an increase in contractility.(ABSTRACT TRUNCATED AT 250 WORDS)

Signal transduction pathways of angiotensin II--induced c-fos gene expression in cardiac myocytes in vitro. Roles of phospholipid-derived second messengers

Angiotensin II (Ang II) causes a rapid induction of immediate-early genes and hypertrophy in the cardiac myocyte. However, the signaling mechanism of Ang II-induced immediate-early gene expression in cardiac myocytes has not been characterized. Therefore, we examined signal transduction of Ang II in neonatal rat cardiac myocytes, using c-fos gene expression as a model system. Transient transfection of c-fos reporter gene constructs indicated that the serum response element is not only required but also sufficient for Ang II-induced activation of the c-fos promoter. Ang II is known to cause an increase in [Ca2+]i. We found that Ang II also causes a small increase in cAMP in cardiac myocytes. However, the Ca2+/cAMP response element of the c-fos gene was not sufficient to confer Ang II responsiveness to the c-fos promoter, and inhibitors of protein kinase A had no effects on Ang II-induced c-fos expression. On the other hand, chelating intracellular Ca2+ with BAPTA-AM inhibited Ang II-induced c-fos expression in a dose-dependent manner, suggesting that Ca2+ is required for Ang II-induced signaling. Measurements of phospholipid-derived second messengers revealed that Ang II increased production of inositol trisphosphate, diacylglycerol, phosphatidic acid, and arachidonic acids, resulting in a sustained increase in protein kinase C activity. This and other evidence suggest that Ang II activates phospholipase C, phospholipase D, and possibly phospholipase A2. All of these second-messenger systems are activated through the AT1 receptor. Pharmacological inhibition of phospholipase C or downregulation of protein kinase C significantly suppressed Ang II-induced c-fos expression. In conclusion, Ang II activates multiple phospholipid-derived second-messenger systems via the AT1 receptor in cardiac myocytes. Among these second-messenger systems, phospholipase C and protein kinase C seem essential for Ang II-induced c-fos gene expression, whereas Ca2+ may play a permissive role. Finally, the "Ang II response element" of the c-fos gene maps to the protein kinase C-dependent portion of the serum response element.

A low affinity, low molecular weight endothelin-A receptor present in neonatal rat heart

1. Addition of endothelin-1 (ET-1) to [3H]-inositol-labelled neonatal rat hearts stimulated the accumulation of [3H]-labelled inositol phosphates (InsP), but only at high concentrations; concentration at half maximum stimulation (EC50) > 0.1 mumol/L). When similar experiments were performed using isolated myocytes, the potency of endothelin-1 was higher and the EC50 value averaged 3.2 +/- 0.5 nmol/L (mean +/- s.e.m., n = 4). 2. The binding affinity of [125I]-endothelin-1 was higher for receptors on isolated cells than for receptors on membranes prepared from intact heart (72 +/- 16 pmol/L compared with 3.9 +/- 0.7 nmol/L, mean +/- s.e.m., n = 4, P < 0.01; Students' t test). 3. Receptors from both sources were cross-linked to [125I]-endothelin-1 and their molecular weights measured using sodium dodecylsulfate gradient polyacrylamide gel electrophoresis (SDS-PAGE). The receptors present on the isolated cells had a higher molecular weight (48 kD) than the receptor on the heart membranes (38 kD).

Stimulation of phosphatidylinositol turnover in adult rat left atria does not involve release of inositol(1,4,5)trisphosphate

1. The turnover rate of inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) in noradrenaline-stimulated adult rat left atria was calculated from changes in specific activity and was found to equal 110 ct/min per mg tissue. In contrast, the isomers of inositol mono- and bisphosphates accumulated at a rate of 508 ct/min per mg. 2. Neomycin, which inhibits release of Ins(1,4,5)P3, inhibited the accumulation of inositol phosphates in noradrenaline-stimulated isolated neonatal cardiomyocytes but did not inhibit accumulation in left atria. 3. These data demonstrate that most of the inositol phosphates which accumulate in adult rat left atria do not derive from Ins(1,4,5)P3. 4. These data are best explained by a model in which noradrenaline stimulation results mainly in the breakdown of phosphatidylinositol(4)monophosphate (PtIns(4)P1) to inositol(1,4)bisphosphate (Ins(1,4)P2). Thus, heart tissue avoids the generation of Ins(1,4,5)P3.

Isolation of adult cardiomyocytes initiates a return of inositol trisphosphate phosphorylating activity

1. We have previously reported that the addition of noradrenaline to [3H]-inositol-labelled adult rat atria or isolated perfused hearts caused the release of inositol-1,4,5-trisphosphate, which was metabolized by dephosphorylation to inositol-4-monophosphate. Inositol-1,3,4,5-tetrakisphosphate and its dephosphorylation products were not detected. 2. In the current study, the addition of noradrenaline to [3H]-inositol-labelled adult rat cardiomyocytes caused the release of inositol-1,4,5-trisphosphate, which was metabolized in part by phosphorylation to inositol-1,3,4,5-tetrakisphosphate. 3. These results demonstrate that the isolation and culture of rat adult cardiomyocytes initiates enhanced generation of inositol-1,3,4,5-tetrakisphosphate. This change would be expected to enhance the calcium response of the cells to stimulation of alpha 1-adrenoceptors.