Role of adenosine in the depolarization of hypoxic hamster diaphragm membrane in vitro

The resting membrane potential of in vitro hamster diaphragm muscle fibers is depolarized on exposure to hypoxia. It was hypothesized that this depolarization was mediated by adenosine. It was predicted that the treatment of well-oxygenated hamster diaphragm muscle strips in vitro with adenosine or adenosine agonists would depolarize the diaphragm fiber membrane. Furthermore, resting membrane potential of hypoxic diaphragm fibers would be repolarized by (1) the removal of adenosine by the enzyme adenosine deaminase (ADA), or (2) the addition of an adenosine antagonist, BW A1433. Adenosine (10(-4) M) depolarized the membrane by 8 +/- 1 mV (p < 0.001). The adenosine agonist cyclopentyladenosine, which has predominantly A1 receptor affinity, depolarized the membrane from -75.4 +/- 5.6 mV to -68.9 +/- 5.7 mV (p < 0.001). The A2 adenosine receptor agonist 5'-N-ethylcarboxamide adenosine did not cause a significant depolarization. The addition of ADA (2 unit/ml) to hypoxic muscle returned the resting membrane potential to that of well-oxygenated fibers, p < 0.001 versus hypoxia. BW A1433 (3 x 10(-7)) also restored the membrane potential of hypoxic muscle fibers from -72 +/- 1 mV to -79 +/- 1 mV (p < 0.001). These observations suggest that adenosine via the A1 adenosine receptor mediates the hypoxic depolarization of in vitro hamster diaphragm muscle. A direct effect of adenosine on muscle membrane has not been described previously.