MODULATION OF RESISTANCE ARTERY FORCE GENERATION BY EXTRACELLULAR CA2+

We tested the hypothesis that increasing extracellular Ca2+ (Ca-o) over a physiological concentration range depresses vascular smooth muscle force generation by altering the intracellular Ca2+ (Ca-i)-force relationship. Mesenteric resistance arteries were isolated from Wistar rats; Ca-i and isometric force were measured using a fura-based method and wire myography. Vessels were depleted of releasable Ca-i by repeated contraction with norepinephrine; Ca-o was then cumulatively added back from 0.025-2.5 mM in the preszence of an agonist. With norepinephrine, serotonin, prostaglandin F-2 alpha, and K+, Ca-o from 0.025 to 0.8 mM induced a graded increase in Ca-i and active stress. With the receptor agonists but not K+ raising Ca-o from 0.8 to 1.6 mM and from 1.6 to 2.5 mM decreased active stress to 82 +/- 6 and 54 +/- 6% of maximum, respectively, P < 0.05. Although there was a transient decrease in Ca-i in response to both 1.6 and 2.5 mM Ca-o, steady-state Ca-i only decreased significantly in response to 2.5 mM Ca-o (85 +/- 3% of maximum). Inhibition of the sarcoplasmic reticulum Ca2+-adenosinetriphosphatase with 1 mu M thapsigargin had no effect on the decrease in force induced by high Ca2+. The decrease in active stress induced by 1.6 and 2.5 mM Ca-o was inhibited by Ca2+ channel antagonists and by blockade of Ca2+-activated K+ channels with charybdotoxin (with 1.6 mM Ca-o control tension = 67 +/- 10% of maximum vs. charybdotoxin = 99.2 +-/ 1%, P < 0.05; n = 9). Inhibition of protein kinase C with bis-indolylmaleimide or H-7 depressed the contractile response to increasing Ca-o but had little effect on the decrease in force induced by 1.6 and 2.5 mM Ca-o. We conclude that Ca-o limits force generation by depressing myofilament Ca2+ sensitivity and decreasing Ca-i secondary to activation of a Ca2+-sensitive K+ channel.