Effects of 8-bromo cyclic GMP and verapamil on depolarization-evoked Ca2+ signal and contraction in rat aorta.
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 GMP on Ca2+ movements and contraction in rat isolated endothelium-denuded aorta stimulated by KCl depolarizing solution using the permeant analogue 8-bromo cyclic GMP (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 microM) 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 microM); 45Ca2+ influx in KCl-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 BrcGMPdid 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 toBrcGMP 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.
Salomone
S
S
Laboratoire de Pharmacologie, Université Catholique de Louvain, Bruxelles, Belgium.
Morel
N
N
Godfraind
T
T
D000818:Animals; D001011:Aorta / Q000187:drug effects*; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects; D006152:Cyclic GMP / Q000031:analogs & derivatives* / Q000494:pharmacology; D016257:Fura-2; D008345:Manganese / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000502:physiology; D017680:Potassium Compounds / Q000494:pharmacology; D051381:Rats; D014700:Verapamil / Q000494:pharmacology*
D015220:Calcium Channels; D017680:Potassium Compounds; C016276:8-bromocyclic GMP; D008345:Manganese; D014700:Verapamil; D006152:Cyclic GMP; D002118:Calcium; D016257:Fura-2