ADENOSINE DISCRIMINATES BETWEEN THE CAFFEINE AND ADENINE-NUCLEOTIDE SITES ON THE SHEEP CARDIAC SARCOPLASMIC-RETICULUM CALCIUM-RELEASE CHANNEL

Calcium-release channels of sheep cardiac sarcoplasmic reticulum were incorporated into phosphatidylethanolamine bilayers and single channel currents were recorded under voltage-clamp conditions. The effect of adenosine on single channel conductance and gating was investigated, as were the interactions between adenosine and caffeine and adenosine and alpha,beta-methylene ATP. Addition of adenosine (0.5-5 mM) to the cytosolic but not the luminal side of the membrane increased the open probability of single calcium-activated calcium-release channels by increasing the frequency and duration of open events, yielding an EC(50) of 0.75 mM at 10 mu M activating Ca2+. Addition of 1 mM caffeine potentiated the effects of adenosine at 10 or 100 mu M-activating cytosolic calcium, but had no effect on the inability of adenosine to activate the channel at 80 pM calcium, suggesting discrete sites of action on the calcium-release channel for adenosine and caffeine. In contrast, addition of 100 mu M alpha,beta-methylene-ATP decreased single channel open probability in the presence of adenosine, suggesting that these compounds act on the same site on the channel. Activation of single channel opening by adenosine, or by adenosine together with caffeine, had no effect on single channel conductance or the Ca2+/Tris(+) permeability ratio. Channels activated by adenosine were characteristically modified by ryanodine and blocked by mu M ruthenium red or mM magnesium. These results show that adenosine activates the sheep cardiac sarcoplasmic reticulum Ca2+-release channel by increasing the frequency and duration of open events in a Ca2+-dependent manner. The receptor site on the channel for adenosine is distinct from that for caffeine but probably the same as that for adenine nucleotides.