Abstract
Junctional sarcoplasmic reticulum (SR) has been identified in microsomes from canine ventricular muscle by the presence of calsequestrin and ryanodinesensitive Ca 2+ release channels. These properties, however, are not common to cardiac cells from all species. Seiler et al (1) have recently described a high M r polypeptide in canine junctional SR similar to the spanning protein subunits of skeletal muscle triads. We now report the existence of a polypeptide with the same mobility in SR from rabbit ventricular muscle and show that those cardiac membranes can associate with transverse (T−) tubules from rabbit skeletal muscle in K cacodylate medium. We propose that this polypeptide and the reaction with T-tubules be considered as criteria for the identification of cardiac junctional SR.
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