INACTIVATION OF L-TYPE CA CHANNELS IN EMBRYONIC CHICK VENTRICLE CELLS- DEPENDENCE ON THE CYTOSKELETAL AGENTS COLCHICINE AND TAXOL

This article shows that colchicine and taxol strongly influence the ki netics of L-type Ca channels in intact cardiac cells, and it suggests a mechanism for this action. It is known that colchicine disassociates microtubules into tubulin, and that taxol stabilizes microtubules. We have found that colchicine increases the probability that Ca channels are in the closed state and that taxol increases the probability they are in the open state. Moreover, taxol lengthens the mean open time o f Ca channels. In this regard, taxol is similar to Bay-K 8644; however , Bay K works on inside-out patches, but taxol does not. Neither colch icine nor taxol alters the number of Ca channels in a patch. We have q uantified these results as follows. It is known that L-type channels i n embryonic chick heart ventricle cells have voltage- and current-depe ndent inactivation. In 10 mM Ba, channel conductance is linear in the range -10 to 20 mV. The conductance is 12 +/- 1 pS, and the extrapolat ed reversal potential is 42 +/- 2 mV (n = 3). In cell-attached patches , inactivation depends on the number of channels. One channel (holding at -80 mV and stepping to 0 mV for 500 ms) shows virtually no inactiv ation. However, three channels inactivate with a time constant of 360 +/- 20 ms (n = 6). In similar patches, colchicine (80 mu M for 15 min) decreases the inactivation time constant to 162 +/- 33 ms (n = 4) and taxol (50 mu M for 10 min) virtually abolishes inactivation (time con stant 812 +/- 265 ms (n = 4)). We suggest that colchicine and taxol af fect Ca channels through their action on the cytoskeleton, which in tu rn regulates the effective concentration of inactivating ions near the mouths of channels. An alternate explanation is that free tubulin int eracts directly with Ca channels.