title: Differences between the prejunctional effects of phenylephrine and clonidine in guinea-pig isolated atria.
abstract: The prejunctional effects of clonidine and phenylephrine were studied in guinea-pig isolated atria by means of field stimulation of the sympathetic nerve terminals during the cardiac refractory period, in the presence of 1 microM atropine. Clonidine (10-100 nM) produced a dose-dependent decrease in the stimulus-inotropic response curve; the IC50 for clonidine was increased about 70 times by the pretreatment of the preparations with 1 microM yohimbine. The effect of clonidine was not modified by 0.5 microM prazosin. Unlike clonidine, phenylephrine (1-10 microM) induced a statistically insignificant increase in the contractile force of preparations stimulated at 4 Hz. The inhibitory effect of phenylephrine (1-10 microM) was partially prevented by either 1 microM yohimbine or 0.5 microM prazosin. However, it was antagonized, to about the same degree as that observed with clonidine, by the pretreatment of the preparations with both 1 microM yohimbine and 0.5 microM prazosin. The results seem to indicate that one component of the prejunctional effects of phenylephrine may be mediated by presynaptic alpha-adrenoceptors belonging to the alpha 1-subtype.
mesh_terms: D000818:Animals; D003000:Clonidine / Q000494:pharmacology*; D004558:Electric Stimulation; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects / Q000294:innervation*; D006325:Heart Atria / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D010656:Phenylephrine / Q000494:pharmacology*; D011224:Prazosin / Q000494:pharmacology; D013564:Sympathetic Nervous System / Q000187:drug effects*; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D010656:Phenylephrine; D015016:Yohimbine; D003000:Clonidine; D011224:Prazosin