Carbachol inhibition of Ca2+ currents in ventricular cells obtained from neonatal and adult rats

Increased sensitivity of neonate atrial myocytes to adenosine A1 receptor stimulation in regulation of the L-type Ca2+ current

Adenosine has cardioprotective effects against ischemia, and newborn hearts show high resistance to ischemia. The effects of purinoceptor stimulation by adenosine and ATP on the L-type Ca2+ current (ICa) were examined in atrial cells from neonate and adult rabbits. ICa was measured by the membrane-perforated patch method. Adenosine inhibited the isoproterenol-stimulated ICa more potently in neonate cells than in adult cells. The high sensitivity of neonate myocytes to adenosine was accompanied not only by an increased maximum response but also by a lower IC50 concentration. ATP also inhibited isoproterenol-stimulated ICa. The effect of ATP on neonate cells was stronger than that on adult cells at high concentrations (greater than or = 100 microM). The effect of adenosine was antagonized by an A1 adenosine receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). DPCPX or an ecto-5'-nucleosidase inhibitor (alpha,beta-methylene-ADP) blocked most (approximately 60%) of the effect of ATP (30 microM), and co-addition of DPCPX and suramin (P2 receptor blocker) abolished the effect of ATP. Suramin alone did not reduce the effect of ATP significantly in neonate cells. Both the effects of adenosine and ATP were eliminated by pre-treatment with pertussis toxin or by superfusion with forskolin plus 3-isobutyl-1-methylxanthine (IBMX). Inhibitors of the nitric oxide-cyclic GMP pathway did not affect the adenosine inhibition of ICa. In summary, neonatal myocardial cells are highly sensitive to adenosine A1 receptor stimulation. ATP stimulates both the adenosine A1 and P2 receptors. Adenosine A1 receptor stimulation, as a result of hydrolysis of ATP, predominantly mediates the effect of ATP, and the role of P2 receptors in the ATP inhibition of ICa is relatively small in neonate cells. The high sensitivity to adenosine may contribute to the ischemic tolerance of newborn hearts.

Phosphatidic acid increases inositol-1,4,5,-trisphosphate and [Ca2+]i levels in neonatal rat cardiomyocytes

Phosphatidic acid (PA), which can be synthesized de novo, or as a product of phosphatidylcholine hydrolysis and/or phosphorylation of 1,2-diacylglycerol (DAG), mediates diverse cellular functions in various cell types, including cardiomyocytes. We set out to characterize the effect of PA on intracellular free calcium ([Ca2+]i) and inositol-1,4,5-trisphosphate (IP(3)) levels in primary cultures of neonatal rat cardiomyocytes. Addition of PA led to rapid, concentration and time dependent increases in both IP(3) and [Ca2+]i levels in adherent cells. There was strong correlation in the concentration-response relationships between IP(3) and [Ca2+]i increases evoked by PA. Incubation with the sarcoplasmic reticulum (SR) Ca2+ pump inhibitor, cyclopiazonic acid (CPA), significantly attenuated the PA evoked [Ca2+]i increase but had no significant effect on IP(3) accumulation. The phospholipase C (PLC) inhibitor, D-609, attenuated both IP(3) and [Ca2+]i elevations evoked by PA whereas staurosporine (STS), a potent and non-selective PKC inhibitor, had no significant effect on either. Another PLC inhibitor, U73122, but not its inactive analog, U73343, also inhibited PA evoked increases in [Ca2+]i. Depletion of extracellular calcium attenuated both basal and PA evoked increases in [Ca2+]i. The PLA(2) inhibitors, bromophenylacyl-bromide (BPB) and CDP-choline, had no effect on PA evoked [Ca2+]i responses. Neither the DAG analog, dioctanoylglycerol, nor the DAG kinase inhibitor, R59949, affected PA evoked changes in [Ca2+]i. Taken together, these data indicate that PA, in a manner independent of PKC, DAG, or PLA(2), may enhance Ca2+ release from IP(3) sensitive SR Ca(2+) stores via activation of PLC in neonatal rat cardiomyocytes.