Sodium ions selectively eliminate the fast component of guanosine cyclic 3',5'-phosphate induced calcium release from bovine rod outer segment disks

Guanosine cyclic 3',5'-phosphate (cGMP) induced Ca2+ release from bovine rod outer segment (ROS) disks showed two kinetic components that could be distinguished in three ways: (1) The slow component (half-rise time of about 30 s) was blocked by 1-cis diltiazem [cf. Koch, K. W., & Kaupp, U. B. (1985) J. Biol. Chem. 260, 6788-6800], whereas the fast component (half-rise time of less than 1 s) was not affected by 1-cis diltiazem. (2) The slow component required the presence of alkali cations, whereas the fast component did not. (3) Preincubation with Na+ (50 mM) selectively eliminated the fast component, whereas the slow component was not affected. The action of Na+ appeared to be caused by Na-Ca exchange removing Ca2+ from a pool that can also be accessed by cGMP. The slow component of cGMP-induced Ca2+ release was not affected by Na+ and, hence, appears to reside in disks that do not contain a functional Na-Ca exchanger. The local anesthetic tetracaine blocked both the slow and fast component of cGMP-induced Ca2+ release from bovine ROS disks.