The effects of calcium and magnesium ions, temperature and repetitive stimulation on inhibitory junctional transmission in smooth muscle of guinea-pig small intestine.

1 The effects of calcium and magnesium ions and temperature on the peak amplitude of the nonadrenergic, noncholinergic inhibitory junction potential, evoked by a single stimulus, or paired transmural stimuli, were examined in the circular muscle of guinea-pig small intestine.2 The peak amplitude of the inhibitory junction potential (i.j.p.) could be decreased by lowering the external concentration of calcium or by raising the external magnesium concentration (at 25 degrees C).3 At 25 degrees C, the second of a pair of i.j.ps was larger than the first at short intervals (<0.2 s), but smaller at larger intervals (0.2 to 20 s). Enhancement of the second (test) response decayed exponentially with a time constant of 87 ms. Depression of the test i.j.p. was maximal at 0.4 s and then recovered exponentially with a time constant of 11 s.4 In low calcium or high magnesium solution, depression of the test i.j.p. decreased without any change in the rate of recovery from depression.5 Despite the slow rate of recovery from depression after a single conditioning response, transmitter output could be maintained during low-frequency repetitive stimulation.6 The peak amplitude of the i.j.p. increased as the temperature was raised to 35 degrees C (Q(10) = 1.5).7 In contrast to the neuromuscular junction, the depression of the second of a pair of i.j.ps decreased as the temperature was raised. At 35 degrees C the test i.j.p. was larger than the conditioning i.j.p. for most stimulus intervals (1 to 20 s).8 The results suggest that the rate of replenishment of the store of inhibitory transmitter is sensitive to both temperature and repetitive stimulation.