[Permeability of sarcoplasma reticulum membrane for monovalent cations]

Potassium, sodium and Tris ion permeability of sarcoplasmic reticulum (SR) vesicles was investigated by the method of potential-sensitive probes. Membrane potentials were generated by changing the ionic environment of vesicles and were measured using the probe 3,3'-dipropyl-2,2'-thiodicarbocyanine (diS-C3-(5)). It was shown that 2/3 of the vesicles (Type I) were permeable to K+ and Na+ while the remaining 1/3 (Type II) were not. Both types of vesicles were relatively impermeable to Tris+. The permeability coefficient for Tris+ was estimated to be of the order of 10(-8) cm/sec. The K+ and Na+ permeability coefficients for Type I vesicles were estimated to be greater than 10(-6) sm/sec. A likely physiological function of the K+, Na+-permeable pathways in SR would be to minimize charge and osmotic effects during Ca2+ uptake and release.