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(Circulation. 1997;96:3761-3765.)
© 1997 American Heart Association, Inc.

Articles

Role of the Diadic Cleft in Myocardial Contractile Control

G.A. Langer, MD; ; A. Peskoff, PhD

From the Departments of Physiology and Medicine, Cardiovascular Research Laboratory, UCLA Center for Health Sciences, Los Angeles, Calif.

Correspondence to Dr G.A. Langer, Departments of Physiology and Medicine, Cardiovascular Research Laboratory, MRL-3645, UCLA Center for Health Sciences, 675 Circle Dr S, Los Angeles, CA 90095-1760. E-mail glenn@cvrl.ucla.edu

Key Words: calcium • contractility • muscles • diadic cleft • sarcoplasmic reticulum

	Introduction

 
The diadic cleft space is the region of the cell, in mammalian heart, between the JSR membrane and the inner leaflet of the T-tubular SL membrane. As results accumulate from various laboratories, the role of the cleft region in regulation of the calcium movements of the cell seems to be of considerable significance. Much remains to be learned about the region, but enough is currently known to warrant a brief perspective at this time.

	Contractile Control: Skeletal and Cardiac

 
In terms of calcium control of contraction, it is useful initially to compare this control in skeletal and cardiac muscle, because both demonstrate cleft structure: diadic in the heart, triadic in skeletal muscle. A simple experiment, which attracted little attention at the time, clearly showed that the process of excitation-contraction coupling is very different in skeletal and heart muscle. Armstrong et al¹ showed that single fibers from frog semitendinosus muscle continued to contract for 20 minutes or longer when perfused with zero calcium plus EGTA ([Ca]₀<10^-8 mol/L). By contrast, removal of calcium from the perfusate bathing ventricular cells completely eliminates contraction within the period between two beats.² Just before the Armstrong study, Endo et al³ demonstrated the process of CICR in skinned skeletal muscle fibers. Ironically, he later found⁴ that the process occurred only in heavily calcium-loaded or caffeine-treated skeletal SR and was not physiological for this tissue. Fabiato,^{5 6} however, in a series of seminal studies, later established that CICR played a physiologically crucial role in heart muscle. It then became obvious that excitation-contraction coupling . . . [Full Text of this Article]