The sources of Ca2+ for muscarinic receptor-induced contraction in the rat ileum.

The contractile responses obtained by activation of different muscarinic receptor subtypes in the longitudinal muscle of the rat ileum and especially the responses of this muscle to acetylcholine in a Ca(2+)-free medium have been investigated. In Ca(2+)-containing solution, acetylcholine elicited similar concentration-dependent contractile responses in the duodenum, jejunum and ileum strips of the rat intestine. The response to a maximal concentration of the agonist (1 microM) consisted of a rapid phasic response followed by a slower tonic one. Nifedipine completely relaxes or inhibits the sustained response and only partially diminishes the phasic one, which suggests that the phasic contraction depends on the release of internal Ca2+ entry from the extracellular space through voltage-dependent Ca2+ channels, but that the tonic contraction only depends on the influx of the external ion. In Ca(2+)-free medium, acetylcholine (1 microM) induced phasic contractions that depend on the release of this ion from internal stores. Participation of different subtypes of receptors (M1, M2 and M3) in these responses depends on the inhibitory action shown by methoctramine, 4-DAMP and atropine but not by pirenzepine in two different experimental models.

In vivo recovery of alpha 1-adrenoceptors in rat myocardial tissue after alkylation with phenoxybenzamine.

The rate of recovery of rat myocardial alpha 1-adrenoceptor density and responsiveness after in vivo block with phenoxybenzamine (1 mg/kg, i.p.) have been investigated by measuring [3H]prazosin binding, and noradrenaline-stimulated [3H]inositol phosphate production. Repopulation of alpha 1-adrenoceptors was monoexponential, with a t1/2 of 33 h; functional recovery was also monoexponential, with t1/2 of 28 h. Furthermore, our results clearly demonstrate the absence of a receptor reserve for alpha 1-adrenoceptors mediating noradrenaline-stimulated phosphoinositide breakdown in rat myocardial tissue. These observations indicate a close relationship between the density of [3H]prazosin binding sites and the ability of alpha 1-adrenoceptors to respond to noradrenaline. Moreover, based on competition curves for inhibition of specific [3H]prazosin by WB-4101 to rat myocardial membranes 48 h and 7 days after the administration of phenoxybenzamine, the results suggest that rat myocardial membranes contain both alpha 1-adrenoceptors subtypes, i.e., alpha 1A and alpha 1B, in an approximate ratio of 20:80, and this relative ratio does not seem to be altered during the recovery process.

Alpha 1-adrenoceptor subtypes and inositol phosphates production in heart ventricles of spontaneously hypertensive rats.

To study potential cardiac alpha 1-adrenoceptor changes in the stage of established hypertension, [3H]prazosin binding characteristics and norepinephrine (NE) stimulated production of inositol phosphates in the ventricular tissue of spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto control rats (WKY) were determined. The maximum number of cardiac [3H]prazosin binding sites was significantly reduced in SHR as compared with WKY rats, but the Kd values were similar in both groups of animals. Distribution of the [3H]prazosin-labeled high- and low-affinity binding sites for WB 4101 was similar in SHR and WKY rats. Pretreatment of ventricular heart membranes with the irreversible alpha 1B-adrenoceptor-selective antagonist chloroethylclonidine resulted in the same percentage of loss of specific [3H]prazosin binding in both groups of rats. Together, these results showed a similar ratio of alpha and alpha 1B-adrenoceptor subtypes in SHR and WKY rats. In addition, [3H]inositol phosphate accumulation in response to cardiac alpha 1-adrenoceptor stimulation by NE was significantly reduced in SHR as compared with WKY rats although the EC50 values were not significantly different in the two strains of animals. Thus, we conclude that in an established stage of hypertension there is a specific loss in the number of cardiac alpha 1-adrenoceptors with consequently reduced inositol phosphate turnover. These results are compatible with the reported decrease in the positive inotropic effect mediated through myocardial alpha 1-adrenoceptors in spontaneous hypertension.

Inhibition of calcium entry induced by cularines and isocrasifoline in uterine smooth muscle.

The effects of nifedipine, papaverine and four benzylisoquinoline alkaloids (cularine, cularidine, celtisine and isocrasifoline) were studied in isolated rat uterus in order to clarify the mechanism of their relaxant action. All the compounds tested completely relaxed KCl-induced contractions and totally or partially inhibited oxytocin-induced rhythmic contractions. Only papaverine acted intracellularly, promoting relaxation of contractile responses induced by oxytocin or vanadate in a Ca(2+)-free medium. In spite of the structural relationship between papaverine and the other alkaloids, the mechanism of their relaxant action is not the same. The activities of cularine derivatives and of isocrasifoline were similar to that of nifedipine.