Ryanodine and the adrenergic, purinergic stimulation in the rat vas deferens smooth muscle: Functional and radioligand binding studies

Abstract

The distribution of [3H]ryanodine binding in subcellular fractions isolated from rat vas deferens (RVD) paralleled that of NADPH cytochrome C reductase activity indicating an endoplasmic reticulum origin of the binding sites. Scatchard analysis of [3H]ryanodine binding indicated an homogenous site with a K(d) value of 6.4 nM. The maximum number of ryanodine binding sites was 488 fmol of [3H]ryanodine per milligram of protein. Norepinephrine (NE) or ATP endogenously released after electrical field stimulation (tetrodoxin-sensitive responses), both produced a biphasic contraction of the RVD when the action of the other was blocked. When NE was the agonist (prazosin-sensitive response), the initial transient contraction was suppressed by 30 μM ryanodine whereas the secondary component was abolished by 2 μM nifedipine. When ATP was the agonist (P2 tachyphylaxis-sensitive response), both phases of the contraction were suppressed by 2 μM nifedipine. However, the initial phasic component of the contraction induced by endogenously released ATP was also inhibited by 30 μM ryanodine except at high stimulation frequency (10 Hz). Exogenously added NE and α,β methylene ATP produced concentration-dependent contractions of the RVD. The effect of both agonists was inhibited by 2 μM nifedipine whereas 30 μM ryanodine had little effect except at high concentrations of NE. We conclude that ryanodine binding sites reside in RVD endoplasmic reticulum. There was a lack of uniformity in the effect of ryanodine and nifedipine against alpha adrenoceptor stimulation and purinoceptor stimulation indicating a difference in the stimulation-contraction coupling process between these two modes of stimulation.