Altered alpha 1-adrenoceptor subtypes mediated cardiac function after treatment of propranolol to rats.

AIM

To study inotropic and chronotropic effects mediated by alpha 1A- and alpha 1B-adrenoceptors after 5-d propranolol (Pro) treatment.

METHODS

The positive inotropic and chronotropic effects mediated by alpha 1A and alpha 1B subtypes were determined on isolated left ventricular papillary muscles and right atrium in Pro- and NaCl-treated rats.

RESULTS

The basic contractility of papillary muscles induced by phenylephrine (Phe) was 90 +/- 18 mg in Pro-treated rats and 53 +/- 17 mg in control group (P < 0.05). The increment on force of contraction was 20 +/- 12 mg in Pro-pretreated rats and 5 +/- 5 mg in NaCl-treated rats (P < 0.05). After preincubated with chloroethylclonidine, the increment on force of contraction was reduced in Pro-treated rats, but was not much changed in control group. Phe in presence of 5-methylurapidil induced positive inotropic effect with 13 +/- 5 mg in Pro-treated group, but not in NaCl-treated rats. Under the normal and the inhibited cardiac state, the maximal increment in beat rate mediated by alpha 1B showed no difference between the Pro-treated and NaCl-treated rats.

CONCLUSION

After chronic treatment of Pro, alpha 1-adrenoceptor-mediated positive inotropic effect in rat heart was improved, which was mainly induced by stimulation of alpha 1B when beta-adrenoceptors were blocked.