The influence of hypothermia on P2 receptor-mediated responses of frog skeletal muscle

The contractile responses of isolated Rana ridibunda frog sartorius muscle contractions evoked by electrical field stimulation (EFS) were studied at three temperature conditions of 17, 22 and 27 °C. Temperature-dependent increase of muscle contractility was found. ATP (10-100 μM) concentration dependently inhibited the electrical field stimulation-evoked contractions of sartorius muscle at all three temperatures; this effect was significantly more prominent at a temperature of 17 °C than at other two temperatures. Adenosine (100 μM) also caused inhibition of electrical field stimulation-evoked contractions which was statistically identical at all three temperature conditions tested. A P2 receptor antagonist, pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS, 10 μM) reduced the inhibitory effect of ATP at all three temperatures but did not affect inhibitory action of adenosine. In contrast, 8-(p-sulfophenyl)theophylline (8-SPT, 100 μM), a nonselective P1 receptor antagonist, abolished inhibitory effects of adenosine at all three temperature conditions but did not antagonize inhibition caused by ATP. In electrophysiological experiments, ATP (100 μM) and adenosine (100 μM) temperature dependently reduced end-plate currents recorded in sartorius neuromuscular junction by voltage-clamp technique. The inhibitory effects of both agonists were enhanced with the decrease of temperature. 8-SPT (100 μM) abolished the inhibitory effect of adenosine but not ATP on end-plate currents. Suramin (100 μM), a nonselective P2 receptor antagonist, inhibited the action of ATP but not adenosine, while PPADS (10 μM) had no influence on the effects of either ATP or adenosine. It is concluded from this study that the effectiveness of P2 receptor-mediated inhibition of frog skeletal muscle contraction in contrast to that of adenosine is dependent on the temperature conditions.