Strontium uptake during the different modes of contraction in the smooth muscle of rabbit aorta, rat aorta and guinea-pig taenia coli.

Effects of Sr substitution for Ca on the contractions due to Ca influx in the aortae of rabbit and rat and the guinea-pig taenia coli were examined in order to characterize the pathways of Ca influx activated by either high K or receptor agonists. In normal solution containing Ca, high K increased both muscle tension and 45Ca uptake in rabbit and rat aortae and guinea-pig taenia. In Sr-substituted solution, high K also induced contractions accompanied by the increases in 89Sr uptake. Noradrenaline induced sustained contraction and increase in 45Ca uptake in normal solution whereas it induced only small contraction accompanied by small increase in 89Sr uptake in Sr-substituted solution in the aortae. Histamine increased neither muscle tension nor 89Sr uptake in Sr-substituted solution although it increased muscle tension as well as 45Ca uptake in normal solution in taenia. Ca and Sr both induced contraction in saponin-treated smooth muscles. The concentration of Sr to induce half maximum contraction was approximately 20 times higher than that for Ca in rabbit and rat aortae or 62 times higher in guinea-pig taenia. From these results, it is suggested that Ca and Sr enter the smooth muscle cells through K-activated channels and induce contraction whereas noradrenaline- or histamine-activated channels are less permeable to Sr than to Ca.