Different Ca(2+)-sensitivity in phasic and tonic types of smooth muscles

Abstract

Since the development of intracellular Ca2+ indicators, such as aequorin, fura-2 and indo-1, it became possible to examine the relationship between cytosolic Ca2+ levels ([Ca2+]i) and muscle contraction in various types of smooth muscles. In addition, the use of bacterial alpha-toxin and saponin, beta-escine, enabled us to make permeabilized muscle in which the receptor-coupled signal transduction system remains intact. Using these techniques, it was found that muscle contraction does not always parallel with [Ca2+]i. A typical example of such dissociation is seen in rat aorta which is classified as 'tonic muscle'; receptor agonists induce greater contraction than a high concentration of K+ at a given [Ca2+]i. Another example observed in a 'phasic muscle' of canine antrum is a temporal change in Ca2+ sensitivity; Ca2+ sensitivity initially increases and then decreases during the spontaneous rhythmic contractions. These results suggest that smooth muscle regulation is not explained solely by the classical Ca(2+)-dependent 'myosin phosphorylation theory'.