Effect of volatile anesthetics with and without verapamil on intracellular activity in vascular smooth muscle

Background: Although halothane and isoflurane inhibit receptor agonist-induced smooth muscle contraction by inhibiting Ca2+ influx via the L-type voltage-dependent Ca2+ channels, their effects on pharmacomechanical coupling remained to be clarified, The intracellular action of both anesthetics was studied during agonist-induced contractions using the Ca2+ channel blocker verapamil. Methods: Isolated spiral strips of rat thoracic aorta with endothelium removed were suspended for isometric tension recordings in physiologic salt solution, Cytosolic concentration of Ca2+ ([Ca2+](i)) was measured concomitantly using fura-2-Ca2+ fluorescence, Muscle contraction was evoked by the receptor agonists with 30 nM norepinephrine or 10 mu M prostaglandin F2 alpha (PGF2 alpha), followed by exposure to halothane, at 0%, 1%, 2%, and 3% or isoflurane, at 2% and 4%. The effects of the anesthetics were compared with those of 0.1-1 mu M verapamil (n = 8 for each condition). To clarify the intracellular action of the volatile anesthetics on agonist-induced contractions, this procedure was repeated for the anesthetics only in the presence of 1 mu M verapamil(n = 8 for each condition). The effects of both anesthetics were also examined in nonreceptor-mediated contractions evoked with a 1-mu M dose of the protein kinase C activator, 12-deoxyphorbol 13-isobutylate, which increases the Ca2+ sensitivity of the contractile elements (n = 8 for each). Results: Halothane, isoflurane, and verapamil suppressed norepinephrine- and PGF2 alpha-induced increases in muscle tension and [Ca2+](i) in a concentration-dependent manner. The Ca2+-tension regression lines suggested that the volatile anesthetics reduced Ca2+ sensitivity of the contractile elements during PGF2 alpha-induced contraction, Pretreatment of the muscle strip with verapamil revealed that halothane and isoflurane released Ca2+ during norepinephrine-induced contraction and that [Ca2+](i)-tension relationship was modulated during PGF2 alpha-induced contractions, Halothane at 2% and 3% and isoflurane at 4% suppressed 12-deoxyphorbol 13-isobutylate-induced increases in muscle tension, whereas they enhanced increases in [Ca2+](i), indicating that both anesthetics suppressed Ca2+ sensitivity during 12-deoxyphorbol 13-isobutylate-induced contraction, Conclusions: Verapamil pretreatment unmasked the intracellular action of the anesthetics, Halothane and isoflurane influenced pharmacomechanical coupling during agonist-induced contraction.