α_1A-Adrenoceptor mediated contraction of rat prostatic vas deferens and the involvement of ryanodine stores and Ca²⁺ influx stimulated by diacylglycerol and PKC

Abstract

1. The present study has investigated the α₁-adrenoceptor subtype mediating contraction of the rat isolated prostatic vas deferens and the possible effector mechanisms involved in this response by use of functional experiments.

2. Contractions to noradrenaline in the rat isolated prostatic vas deferens were antagonized by prazosin (9.4, 1.04±0.19, pA₂ and Schild plot slope), 5-methyl urapidil (8.9, 1.10±0.13), BMY 7378 (6.4, 1.53±0.07) and RS 17053 (8.3, 1.13±0.18). These affinities are consistent with the response being mediated by the α_1A-adrenoceptor subtype.

3. The contraction to noradrenaline at 37°C consisted of an initial phasic response, composed of many rhythmic contractile spikes and a more slowly developing tonic contraction. When the temperature was lowered to 25°C the phasic contraction became a smooth single response which was increased in magnitude.

4. In Ca²⁺-free Krebs solution the tonic contraction to noradrenaline (10⁻⁴ M) was abolished, suggesting that this response was dependent on influx of extracellular Ca²⁺. After 2 min in Ca²⁺-free Krebs solution at 37°C and 25°C the phasic response to noradrenaline (10⁻⁴ M) was 38±2% and 91±4%, respectively, compared with the phasic contraction to noradrenaline (10⁻⁴ M in normal Krebs solution) and after 30 min it was abolished at 37°C and was 7±1% at 25°C. Ryanodine abolished the noradrenaline response in Ca²⁺-free Krebs solution for 2 min at 25°C, while cyclopiazonic acid reduced it to 36±2%.

5. In normal Krebs solution at 25°C the protein kinase C inhibitor calphostin C reduced the tonic contraction to noradrenaline (10⁻⁵ M) from 36±8% to 14±3% compared with the phasic contraction to noradrenaline (10⁻⁴ M). The DAG kinase inhibitor R 59022 increased the contraction following the initial phasic response to a maximum of 107±17% after 35 s, before dropping down to a well maintained contraction which was still greater in magnitude compared with the control. Nifedipine (3×10⁻⁷ M) reduced the tonic contraction from 49±6% to 7±1% but did not reduce the phasic response. Ryanodine (10⁻⁴ M) reduced the phasic contraction from 50±2% to 7±1% and the tonic response from 47±5% to 27±5%.

6. The phorbol ester phorbol-12,13-dibutyrate at 25°C produced a transient contraction of the rat prostatic vas deferens, maximum response (10⁻⁵ M) 48±4%, compared with the maximum tonic response to noradrenaline. The contraction to PDBu (10⁻⁵ M) was reduced to 23±2% by calphostin C (10⁻⁶ M) and to 15±1% by nifedipine (3×10⁻⁷ M) and was abolished after 2 min in Ca²⁺-free Krebs solution.

7. In conclusion, the α_1A-adrenoceptor mediated contraction to noradrenaline of the rat prostatic vas deferens appears to consist of an initial phasic component due to the release of intracellular Ca²⁺ from ryanodine-sensitive stores. These stores are depleted in the absence of extracellular Ca²⁺ and this depletion is slower at 25°C than at 37°C. The phasic contraction is followed by a tonic contraction involving activation of protein kinase C by diacylglycerol and influx of Ca²⁺ through nifedipine-sensitive channels.

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