Regional differences in the contractile and intracellular Ca2+ responses of the guinea‐pig vas deferens to neurotransmitters and excess K+

Abstract

The contractile responses to various concentrations of noradrenaline (NA), adenosine triphosphate (ATP), acetylcholine (ACh) and excess external K+ in the epididymal, middle and prostatic portions of the guinea-pig vas deferens were investigated by measuring the isotonic contraction, and the intracellular Ca2+ concentration ([Ca2+]i) using fura-2 fluorescence. In the epididymal portion, the contractions evoked by each of these agonists were biphasic, comprising a transient followed by a tonic phase. In the middle portion, NA and ACh evoked biphasic contractions, whereas the ATP-induced contraction was an almost monophasic transient. In contrast, in the prostatic portion, only transient contractions were evoked by ACh and ATP, while the NA-induced contraction was oscillatory. The responses to each of the neurotransmitters in the three portions were not affected by pretreatment with TTX. The maximal responses of tonic contraction in response to each of the neurotransmitters were largest in the epididymal portion, decreased in the middle and were almost absent in the prostatic portion. These regional differences in the contractile properties of the vas deferens were also evident upon stimulation with excess external K+. As with the contractile responses, regional differences in the [Ca2+]i increases were observed during exposure to all the stimulants. It is suggested that the regional differences in the contractile responses of the vas deferens to various neurotransmitters and excess external K+ may involve variation in the mechanisms of Ca2+ homeostasis and in the sensitivity of the contractile apparatus to intracellular Ca2+ ions.