Aequorin luminescence during activation of single isolated smooth muscle cells

Abstract

INCREASED cytoplasmic Ca²⁺ supposedly couples excitation to contraction in smooth muscle. But there have been no direct measurements of such an increase and there is little information about the kinetics of changes in intracellular Ca²⁺ during individual contractions of smooth muscle. Thus the basic aspects of excitation–contraction (EC) coupling in smooth muscle are not known with the same degree of certainty that exists for striated muscle¹. For example, there is a long delay between stimulation and contraction in smooth muscle; this delay might result from a slow onset of Ca²⁺ release after a change in surface membrane potential or from the slowness of some other process². If it were possible to correlate measurements of transmembrane potential, intracellular Ca²⁺ and contraction in the same single cell the mechanism of EC coupling in smooth muscle might be better understood. Some progress in this direction has been made by the development of methods for enzymatic isolation of smooth muscle cells and measuring electrical activity and force production or shortening in these single cells^2–6. We report here a further approach to the problem by (1) microinjecting the Ca²⁺ indicator aequorin into smooth muscle and (2) directly measuring changes in intracellular Ca²⁺ during EC coupling in single isolated smooth muscle cells. Cytoplasmic Ca²⁺ evidently began to rise without detectable delay following a single electrical stimulus applied to a cell injected with aequorin and reached a peak well before contraction presumably began. The long delay, therefore, between stimulation and contraction seems inherent in the mechanism by which the contractile proteins of smooth muscle are activated.