Effects of 8‐bromo cyclic GMP and verapamil on depolarization‐evoked Ca2+ signal and contraction in rat aorta

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Abstract
1 The pharmacological action of NO donors is usually attributed to a cellular rise in guanosine 3′:5′-cyclic monophosphate (cyclic GMP), but this hypothesis is based only on indirect evidence. Therefore, we have studied the effects of cyclic BMP on Ca2+ movements and contraction in rat isolated endothelium-denuded aorta stimulated by KC1 depolarizing solution using the permeant analogue 8-bromo cyclic BMP (BrcGMP). Isometric contraction and fura-2 Ca2+ signals were measured simultaneously in preparations treated with BrcGMP and with verapamil. The activation of calcium channels was estimated by measuring the quenching rate of the intracellular fura-2 signal by Mn2+ and by the depolarization-dependent influx of 45Ca2+. 2 Stimulation with 67 mM KCl-solution evoked an increase in cytosolic Ca2+ concentration ([Ca2+]cyt) and a contractile response which were inhibited by pretreatment with verapamil (0.1 μm) or BrcGMP (0.1-1 mM). However, the inhibition of the fura-2 Ca2+ signal was significantly higher with verapamil than with BrcGMP, whereas the contraction was inhibited to a similar extent. 3 When preparations were exposed to K+-depolarizing solution in which the calcium concentration was cumulatively increased, the related increase in fura-2 Ca2+ signal was barely affected by BrcGMP, whereas the contractile tension was strongly and significantly inhibited. 4 Cellular Ca2+ changes were also estimated with 45Ca2+. 45Ca2+ influx in resting preparations was significantly reduced by BrcGMP (0.1 mM) but not by verapamil (0.1 μm); 45Ca2+ influx in KC1-depolarized preparations was reduced by verapamil but was unaffected by BrcGMP. 5 Measurements of Mn2+-induced quenching of the intracellular fura-2 signal showed that BrcGMP did not affect divalent cation entry in K+-stimulated preparations, whereas verapamil concentration-dependently inhibited Mn2+ entry stimulated by K+-depolarization. 6 The present results indicate that BrcGMP did not affect voltage-dependent Ca2+ channel gating in the rat aorta. For a given fura-2 Ca2+ signal, the contraction was lower in preparations exposed to BrcGMP than in the untreated ones, suggesting that the activation of cyclic GMP-dependent kinases reduced the contractile efficacy of calcium. Furthermore, the reduction of depolarization-dependent 45Ca2+ uptake reported with sodium nitroprusside, a NO donor, was not observed with biologically active concentrations of BrcGMP, suggesting that this drug could have additional mechanisms of action, unrelated to activation of protein G-kinase.