Effects of Adenine Nucleotides on the Ca 2+ -Gated Cation Channel in Sarcoplasmic Reticulum Vesicles 1

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Abstract
The effects of nucleotides on the Ca 2+ -gated cation channel in sarcoplasmic reticulum (SR) vesicles were studied by measuring choline influx. The choline influx was measured by following the change in scattered light intensity using the stopped flow technique. ATP enhanced the Ca 2+ -induced choline influx. The activation followed a single-site titration curve with a dissociation constant of 1.0±0.5 mM, independent of the Ca 2+ concentration. ATP seems to increase the pore radius or number of channels without affecting the gating mechanism of the Ca 2+ -gated cation channel. ADP, AMP, and adenine enhanced the choline transport in a manner similar to ATP, but cAMP, ITP, UTP, CTP, and GTP did not. The apparent dissociation constants and the maximal activations were as follows: ATP 1.0 mM, 28-fold; ADP 0.9 mM, 18-fold; AMP 0.6 mM, 7-fold, and adenine 0.4 mM, 4-fold. Adenine and AMP behaved as a competitive inhibitor for the activation by ATP. These results are consistent with the Ca 2+ -induced Ca 2+ release observed in skinned muscle fiber and isolated SR.