Fundamental & clinical pharmacology
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Fundam Clin Pharmacol · Jan 1990
Whether phenylephrine exerts inotropic effects through alpha- or beta-adrenoceptors depends upon the relative receptor populations.
Phenylephrine produced concentration-related positive inotropic responses in isolated left atria and papillary muscles of guinea-pigs and rats. In rat tissues, these responses were unaffected by propranolol but antagonized by prazosin and therefore mediated via alpha 1-adrenoceptors. The alpha 1-adrenoceptor agonist methoxamine also exerted positive inotropic effects in these rat tissues. ⋯ This explains why the phenylephrine responses were beta-adrenoceptor-mediated in guinea-pig papillary muscles. In the left atria of guinea-pigs, which displayed both alpha- and beta-adrenoceptor-mediated responses, the densities of alpha- and beta-adrenoceptor binding sites were similar. Thus, phenylephrine exerts positive inotropic effects through alpha- or beta-adrenoceptors depending upon their relative densities.
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The effect of atropine (1-10 micrograms . kg-1) on neuromuscular transmission in humans was studied by analysing its effects on the amplitude of indirectly-elicited twitch (0.2 Hz) and tetanic (50 and 100 Hz for 1 s duration) contractions. Six patients, free from any neuromuscular disorders, undergoing orthopaedic surgery, were included in the present study. The patients received either no premedication or the oral benzodiazepine, temazepam, 30 mg 1-2 h pre-operatively. ⋯ At the same time, heart rate and blood pressures (systolic and diastolic) were recorded. The results showed that atropine enhanced the tetanic contractions, elicited at 50 and 100 Hz for 1 s duration, by 27 +/- 1.2% (50 Hz and atropine, control 220 +/- 13 g tension), and by 43 +/- 7% (100 Hz and atropine, control 333 +/- 26 g tension) at the 5 min intervals (mean +/- S. E., n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)