Regulation of cardiac calcium current by NO and cGMP-modulating agents

Several effects of nitric oxide (NO) on the control of L-type calcium curre nt (I-Ca) and of calcium handling in cardiomyocytes have been described. Ca rdiomyocytes have been shown to express in different conditions all types o f nitric oxide synthases (NOS), but the role of NO in the regulation of cal cium current remains controversial. Previously, we have shown in guinea pig ventricular cells a stimulatory effect of NOS inhibitors on I-Ca. Here we investigate the intracellular mechanisms involved in the putative inhibitor y role of NO on basal I-Ca in ventricular cells. The stimulatory effect of the NOS inhibitor NG-monomethyl-L-arginine (L-NMMA)(1 mM) was present also in calcium transient measurements, but only after a preincubation with L-ar ginine (L-arg, 0.1 mM). The nitric oxide scavenger 2-phenyl-4,4,5,5-tetrame thylimidazoline-1-oxyl-3-oxide (PTIO, 0.5 mM) increased peak I,, in a simil ar manner to NOS inhibitors in whole-cell voltage-clamp experiments. Also O DQ (1H-[1,2,4]oxidiazolo[4,3-a]quinoxaline-1-one, 0.1 mM), a specific inhib itor of a target of NO, the soluble guanylate cyclase, was able to stimulat e I-Ca. The block of type II phosphodiesterase (cGMP-activated) by EHNA (er ythro-9-[2-hydroxy-3-nonyl]adenine, 30 muM) exerted a similar effect on I-C a as PTIO and ODQ. Carbachol (CCh, 1 muM) was able to revert the stimulator y effect on I-Ca observed with PTIO, ODQ, and EHNA. We propose that the inc rease of basal I-Ca in guinea pig cardiomyocytes previously observed with L -NMMA depends on the removal of a tonic NO inhibition. This increase of I-C a is mimicked by blocking at different steps the cGMP-cascade activated by NO, suggesting a NO-guanylate cyclase mechanism in the basal control of ven tricular calcium current.