Ryanoids change the permeability of potassium channels of locust (Schistocerca gregaria) muscle

The plasma membrane of locust skeletal muscle contains two types of K+ channel; a maxi, Ca2+-activated 170 pS channel (BK channel) and an inward rectifier of 35 pS conductance (IK channel). The effects of ryanodine, 9,21-didehydroryanodine and 9,21-didehydroryanodol on these channels have been investigated. In the concentration domain 10(-9) M to 10(-5) M, ryanodine irreversibly induced a dose-dependent reduction of the reversal potential (Vrev) of the channel currents, measured under physiologically normal K+ and Na+ gradients, i.e. from approximately 60 mV in the absence of ryanodine to approximately 15 mV for 10(-5) M ryanodine. The alteration of K+ channel selectivity was Ca2+ independent. 9,21-Didehydroryanodine and 9,21-didehydroryanodol reduced Vrev, but only to approximately 35 mV during application of 10(-5) M of these compounds. 9,21-Didehydroryanodine also diminished the conductances of the locust K+ channels. The three ryanoids reduced Vrev of a Ca2+-activated, high-conductance channel in inside-out patches excised from mouse interosseal muscle, although the changes were in each case less pronounced than those for the locust K+ channels. Also, the action of 9,21-didehydroryanodine on mouse K+ (BK) channels was restricted to a shift of Vrev. For all three ryanoids, the IC50 coefficients (i.e. the concentration of toxin that gave a 50% reduction in Vrev) for the shifts in Vrev were similar for the locust and mouse muscle K+ channels.