Modulation of the cGMP-activated conductance of the plasma membrane of photoreceptor cells by calmodulin.

The effect of calmodulin on the integral cGMP-activated conductance of ROS plasma membrane has been investigated by using the ''patch-clamp'' techniques (inside-out configuration) Calmodulin increased the apparent Michaelis constant (K-m) from 19+/-1.5 (in the absence of calmodulin) to 33.7+/-5.5 mM. However, it did not influence the Hill coefficient and the maximum conductance of cGMP-channels. Ca2+ ions were required for the calmodulin effect (EC(50) = 452+/-47 nM). The results of our experiments with the use of 8-Br-cGMP and IBMX suggest that the modulating action of calmodulin was not mediated by the ROS phosphodiesterase activation. The action of some calmodulin inhibitors has been tested. On the basis of our study calmodulin is suggested to be very significant for the recovery and light adaptation processes by changing the affinity of the cGMP-channel for agonist.