Cgmp-Dependent Calcium Desensitization Contributes to Vasodilator Response to Nitric Oxide in Rat Middle Cerebral Arteries
We have recently reported that a cGMP-dependent mechanism as well as a cGMP-independent activation of KCa channels secondary to a fall in 20-HETE contributes to the vasodilator response to NO in the cerebral circulation. The present study examines the mechanism for the cGMP-dependent component of the vasodilator response to NO in rat middle cerebral arteries (MCA). Administration of a NO donor, dose-dependently, increased the diameter of serotonin preconstricted MCA to 69.3±5.8% of control (n=6). 8-Br-cGMP (10-8 to 10-4 M) mimicked the effect of NO and increased the diameter of MCA to 58.7±3.6% of control (n=6). Blockade of KCa channels with IBTX (10-7 M, n=6) or depolarization with a 80 mM K+ media (n=6)reduced the vasodilator response of MCA to NO by 70±6%, but they had little effect on the vasodilator response to 8-Br-cGMP. This suggests that activation of K+ channels contribute to the vasodilator response to NO in MCA, but not the response to 8-Br-cGMP. We therefore examined the effects of NO and cGMP on Ca2+-induced contraction of rat MCAs permeablized with α-toxin (10 μg/ml) and ionomycin (10 μM). Elevations in bath Ca2+, from 10-8 to 10-5 M, decreased the diameter of these vessels by 60.9±3.7%. A NO donor, DEA-NONOate (10-6 M, n=6)and 8-Br-cGMP (10-4 M, n=6) both attenuated the vasoconstrictor response to elevations in bath Ca2+ by 60%. Inhibition of guanylyl cyclase with ODQ (10-5 M) prevented the effects of the NO donor, but not 8-Br-cGMP. These results indicate that the cGMP, PKG-dependent component of the vasodilator response of NO in rat MCA is mediated by desensitization of the contractile mechanism to calcium rather than activation of K+ channels.