Differentiation of mechanisms for mobilization of calcium in smooth muscle.

Techniques to dissociate different sites or stores important for Ca2+ entry or release in smooth muscle include washouts of 45Ca in cold La3+ -substituted solutions. Scatchard-coordinate plots of Ca2+ uptake, substitution of Sr2+ for Ca2+, and both desaturation and rate coefficient plots. Rabbit aortic smooth muscle is particularly useful because Ca2+ mobilization components can be clearly separated. Other vascular preparations investigated (e.g., renal vessels, coronary arteries) appear to have similar components, but their relative importance varies. Respiratory smooth muscle also has similar Ca2+ mobilization components, but they are less readily dissociated by techniques employed in vascular smooth muscles. In guinea pig trachea, cold La3+ washouts do not retain cellular Ca2+ as well as in other preparations: use of other experimental approaches including the Ca2+ channel entry stimulator, CGP 28392, can demonstrate different Ca2+ uptake mechanisms for K+ -stimulated and agonist-induced Ca2+ uptake. In rabbit aorta, CGP 28392 potentiates tension increases elicited with lower concentrations of added K+ but has no effect on norepinephrine-induced contraction. A general model illustrating different Ca2+ entry mechanisms present in three types of smooth muscle provides examples drawn from a spectrum of possible variations in smooth muscle specificity for Ca2+ mobilization.