EFFECTS OF BETA-2-AGONISTS ON THE CONTRACTILITY OF FATIGUED CANINE DIAPHRAGM IN-VIVO

We examined the effects of four beta2-agonists, clenbuterol, metaprote renol. procaterol, and albuterol, on the contractility of fatigued can ine diaphragm. Diaphragmatic contractility was assessed from changes i n transdiaphragmatic pressure (Pdi) generated by supramaximal electric al stimulation of the phrenic nerve. Diaphragmatic fatigue was develop ed by applying an inspiratory resistive load to spontaneously breathin g dogs for 30-50 min. Four doses of each beta2-agonist were intravenou sly administered to dogs when the decreased Pdi became stable. Clenbut erol increased Pdi at doses of 10 and 20 mug/kg (P<0.05). Metaproteren ol improved Pdi of the fatigued diaphragm at low stimulation frequenci es in a dose-dependent manner (P<0.05). Procaterol increased Pdi at a low dose but decreased Pdi at higher doses, possibly due to a marked d ecrease in systemic blood pressure. Albuterol did not change Pdi at an y dose. These results suggest that both clenbuterol and metaproterenol have a positive inotropic effect on fatigued canine diaphragm, while procaterol has a negative effect at high doses. Albuterol showed no in otropic effect.