Nitric oxide regulates the calcium current in isolated human atrial myocytes

Abstract

Cardiac Ca2+ current (I(Ca)) was shown to be regulated by cGMP in a number of different species. Recently, we found that the NO-donor SIN-1 (3- morpholino-sydnonimine) exerts a dual regulation of I(Ca) in frog ventricular myocytes via an accumulation of cGMP. To examine whether NO also regulates Ca2+ channels in human heart, we investigated the effects of SIN-1 on I(Ca) in isolated human atrial myocytes. An extracellular application of SIN-1 produced a profound stimulatory effect on basal I(Ca) at concentrations > 1 pM. Indeed, 10 pM SIN-I induced a ≃ 35% increase in I(Ca). The stimulatory effect of SIN-1 was maximal at 1 nM(≃ 2-fold increase in I(Ca)) and was comparable with the effect of a saturating concentration (1 μM) of isoprenaline, a β-adrenergic agonist. Increasing the concentration of SIN-1 to 1100 μM reduced the stimulatory effect in two thirds of the cells. The stimulatory effect of SIN-1 was not mimicked by SIN-1C, the cleavage product of SIN-1 produced after liberation of NO. This suggests that NO mediates the effects of SIN-1 on I(Ca). Because, in frog heart, the stimulatory effect of SIN-1 on I(Ca) was found to be due to cGMP-induced inhibition of cGMP- inhibited phosphodiesterase (cGI-PDE), we compared the effects of SIN-1 and milrinone, a cGI-PDE selective inhibitor, on I(Ca) in human. Milrinone (10 μM) induced a strong stimulation of I(Ca) (≃ 150%), demonstrating that cGI- PDE controls the amplitude of basal I(Ca) in this tissue. In the presence of milrinone, SIN-1 (0.1-1 nM) had no stimulatory effect on I(Ca), suggesting that the effects of SIN-1 and MIL were not additive. We conclude that NO may stimulate I(Ca) in human atrial myocytes via inhibition of the cGI-PDE.