Activation of voltage-independent Ca2+ entry by noradrenaline involves cGMP in vascular myocytes

Stimulation of portal vein myocytes with noradrenaline (NA) in the presence of a voltage-dependent Ca2+ channel blocker, evoked a transient increase in the concentration of free cytosolic Ca2+, due to inositol 1,4,5-trisphosphate mediated Ca2+ release, followed by activation of a Ca2+ entry pathway. Combining patch-clamp and indo-1 measurements we have tested the effects of various pharmacological agents on this Ca2+ entry following NA-induced Ca2+ release in order to determine the mechanism involved. Only the guanylate cyclase inhibitor LY-83583 specifically inhibited the maintained Ca2+ entry during NA stimulation. This inhibition was reversed by dibutyryl cGMP (DB-cGMP) or 8-bromo cGMP. Under control conditions, addition of DB-cGMP to the external solution was without effect. Thapsigargin and caffeine each depleted the intracellular Ca2+ store but did not evoke Ca2+ entry in venous myocytes under control conditions. However, application of DB-cGMP or NA after Ca2+ store depletion induced by caffeine or thapsigargin caused a rise in [Ca2+]i by activation of a Ca2+ entry pathway. The effect of cGMP seems to involve phosphorylation since cGMP-activated protein kinase inhibitors KT-5823 and H-8 blocked the NA-induced Ca2+ entry. Our results thus suggest that the activation of the voltage-independent Ca2+ entry by NA involves an increase in cellular cGMP.