Maturation attenuates the effects of cGMP on contraction, [Ca2+]i and Ca2+ sensitivity in ovine basilar arteries

Abstract

The present study explores the hypothesis that age-related variations in cerebrovascular responses to vasodilators reflect corresponding age-dependent differences in the mechanisms coupling changes in cytosolic cGMP to vasorelaxation. The experiments focused on cGMP's ability to decrease either [Ca²⁺]_i or myofilament Ca²⁺ sensitivity, because both effects can contribute to cGMP-induced vasodilation. Use of the cGMP analog 8-pCPT-cGMP minimized problems associated with limited cell permeation or cGMP hydrolysis. In fetal basilars contracted with 10 μM serotonin, the EC₃₀ for 8-pCPT-cGMP-induced relaxation was 6 μM. In fura-2 loaded fetal basilars, pretreatment with 6 μM 8-pCPT-cGMP significantly depressed the sensitivity of [Ca²⁺]_i to 5HT, and also myofilament sensitivity to calcium, but only in fetal arteries. In fetal basilar arteries contracted with 120 mM potassium, the EC₃₀ for 8-pCPT-cGMP-induced relaxation was 25 μM. In fura-2 loaded ovine arteries, pretreatment with 25 μM 8-pCPT-cGMP had no effect on the ability of graded concentrations of potassium to elevate [Ca²⁺]_i but reduced potassium's ability to induce contraction and attenuated myofilament calcium sensitivity; these latter effects were significant only in fetal arteries. In α-toxin permeabilized preparations, 25 μM 8-pCPT-cGMP significantly depressed both basal- and agonist-stimulated myofilament calcium sensitivity, only in fetal but not in adult basilars. Together, these results demonstrate that: (1) sensitivity to cGMP is greater in fetal than adult sheep arteries independent of method of contraction; (2) cGMP can reduce [Ca²⁺]_i but only in agonist-contracted and not in potassium-contracted arteries; (3) and cGMP attenuates myofilament calcium sensitivity regardless of method of contraction. Overall, the data demonstrate that variations in the ability of cGMP to produce vasodilatation reflect age-, artery-, and agonist-dependent differences in the combination of mechanisms mediating responses to cGMP.