Possible mechanism of the potent vasoconstrictor responses to ryanodine in dog cerebral arteries

Abstract

Isolated cerebral (basilar, posterior communicating and middle cerebral) arteries exist in a partially contracted state. To determine the Ca 2+-buffering function of sarcoplasmic reticulum in the resting state of cerebral arteries, the effects of ryanodine that eliminates the function of sarcoplasmic reticulum, on tension and cellular Ca 2+ level were compared in endothelium-denuded strips of the cerebral, coronary and mesenteric arteries of the dog. The addition of ryanodine to strips with basal tone caused a concentration-dependent contraction, which was significantly greater in the cerebral arteries than in the mesenteric or coronary artery. In the presence of 10 −5 M ryanodine, the caffeine (20 mM)-induced contraction was greatly attenuated in these arteries. After washout, the basal tone was greatly elevated in the cerebral arteries. The elevated tone was abolished by 10 −7 M nifedipine. The ryanodine-induced contractions were also abolished by 10 −7 M nifedipine. Nifedipine itself caused a relaxation from the basal tone in the cerebral arteries, suggesting the maintenance of myogenic tone. The basal Ca 2+ influx in arteries measured after a 5-min incubation with 45Ca was significantly higher in the basilar artery than in the mesenteric artery. The basal Ca 2+ influx was not increased by 10 −5 M ryanodine in either artery. The basal Ca 2+ influx was decreased by 10 −7 M nifedipine in the basilar artery, but was unchanged in the mesenteric artery. These results suggest that: (1) the basal Ca 2+ influx via L-type voltage-dependent Ca 2+ channels was higher in the resting state of the cerebral arteries; (2) the greater part of the higher Ca 2+ influx was buffered by Ca 2+ uptake into the sarcoplasmic reticulum; and (3) therefore the functional elimination of sarcoplasmic reticulum by ryanodine caused a potent contraction in these arteries. Furthermore, the maintenance of myogenic tone in the cerebral arteries suggests that more Ca 2+ enters the smooth muscle cell than the buffering ability of sarcoplasmic reticulum can handle.