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

Abstract

Background: Although halothane and isoflurane inhibit receptor agonist- induced smooth muscle contraction by inhibiting Ca 2+ influx via the L-type voltage-dependent Ca 2+ channels, their effects on pharmacomechanical coupling remained to be clarified. The intracellular action of both anesthetics was studied during agonist-induced contractions using the Ca 2+ 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 Ca 2+ ([Ca 2+](i)) was measured concomitantly using fura-2-Ca 2+ fluorescence. Muscle contraction was evoked by the receptor agonists with 30 nM norepinephrine or 10 μM prostaglandin F2α (PGF2α), 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 μM 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 μM verapamil (n = 8 for each condition). The effects of both anesthetics were also examined in nonreceptor-mediated contractions evoked with a 1-μM dose of the protein kinase C activator, 12-deoxyphorbol 13-isobutylate, which increases the Ca 2+ sensitivity of the contractile elements (n = 8 for each). Results: Halothane, isoflurane, and verapamil suppressed norepinephrine- and PGF2α- induced increases in muscle tension and [Ca 2+](i) in a concentration- dependent manner. The Ca 2+-tension regression lines suggested that the volatile anesthetics reduced Ca 2+ sensitivity of the contractile elements during PGF2α-induced contraction. Pretreatment of the muscle strip with verapamil revealed that halothane and isoflurane released Ca 2+ during norepinephrine-induced contraction and that [Ca 2+](i)-tension relationship was modulated during PGF2α-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 [Ca 2+](i), indicating that both anesthetics suppressed Ca 2+ 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.