Effects of nifedipine and ryanodine on adrenergic neurogenic contractions of rat vas deferens: evidence for a pulse‐to‐pulse change in Ca 2+ sources

1 The effects of nifedipine and ryanodine on the adrenergic component of neurogenic contractions of the rat isolated vas deferens were studied in an attempt to identify the sources of Ca 2+ mediating the contraction. The tissue was electrically stimulated by single pulses or pairs of widely spaced pulses. The purinergic component of contraction was suppressed by the presence of 300 μ m suramin. 2 In Mg 2+ ‐free medium, nifedipine (0.01–10 μ m ) reduced the first and, to a greater extent, the second twitch elicited by two pulses 3 s apart. This pattern of inhibition was observed both in the absence of rauwolscine (when twitch 2 was smaller than twitch 1) and in the presence of 0.1 μ m rauwolscine (when, due to interruption of prejunctional α 2 ‐adrenoceptor‐mediated autoinhibition, twitch 2 was of similar height to twitch 1). Nifedipine reduced only twitch 2 but not twitch 1 in medium containing 1.2 m m Mg 2+ . 3 Single pulses of increasing current strength elicited increasing contraction. Nifedipine reduced contractions by about the same absolute extent at all current strengths, so that the relative contribution of the nifedipine‐sensitive component decreased with increasing current strength. 4 When the pulse interval in a pair was increased from 5 to 60 s, the inhibition by nifedipine of the second twitch was most marked at an interval of 5 s and declined as the interval increased. 5 In contrast to nifedipine, 20 μ m ryanodine reduced the first twitch of a pair to a greater extent than a second twitch 5 s later, so that twitch 2 became greater than twitch 1. The inhibition by ryanodine of twitch 2 increased with increasing pulse interval. 6 In vasa deferentia preincubated with [ 3 H]‐noradrenaline, 1 μ m nifedipine and 20 μ m ryanodine did not change the electrically evoked overflow of tritium, whereas 10 μ m nifedipine increased it. 7 It is concluded that, when the sympathetic axons of the vas deferens are stimulated by a single pulse (or the first pulse of a pair) in Mg 2+ ‐free medium, both Ca 2+ mobilization inside the smooth muscle cells and Ca 2+ entry contribute to the ensuing adrenergic contraction. The relative contribution of Ca 2+ entry is small at maximal stimulus strength but increases with decreasing stimulus strength. When a second pulse follows the first after an appropriate interval, a switch of Ca 2+ sources occurs: intracellular Ca 2+ mobilization is decreased during twitch 2, whereas Ca 2+ entry is increased.