Calcium pools in the contraction of arterial smooth muscle induced by several agonists

After perfusing the vascular circuit of the mesenteric artery of the rat with a Ca-free solution for 20 min, extracellular Ca was depleted, and the vasoconstrictor effect of high K solutions was abolished, while the vasopressin response was reduced by 40%, serotonin by 30% and noradrenaline and adrenaline responses by about 20%. When the Ca-free perfusion was prolonged for 2 h or more, the vascular responses to the agonists were further reduced due to depletion of cellular Ca. Successive doses of each agonist injected at 1 min intervals showed a decline of the responses, that recovered when 4 min or more were allowed to pass after the last injection. This reversible desensitization or tachyphylaxis was rapidly abolished by Ca. Noradrenaline and adrenaline showed crossed tachyphylaxis, which depressed the responses to serotonin and vasopressin. Tachyphylaxis to serotonin and vasopressin did not depress the response to the other two agonists. These findings support the conclusion that not only do noradrenaline and adrenaline share a Ca pool, but that so do serotonin and vasopressin. These two agonists each require a separate Ca pool.