Abstract
We studied the role of sarcolemmal α- and β-adrenoceptors activation in the effects of cocaine on the positive force staircase in isolated guinea pig atria. The preparations were superfused with Tyrode's solution at 31 °C while attached to a force transducer to measure peak tension developed (PTD), maximum velocity of development of tension (V max T) and time to peak tension (TPT). The positive force staircase was not affected by propranolol or phentolamine, but it was abolished by nifedipine. Cocaine 1 mg l (2.9 μM) enhanced PTD and V max T, while TPT remained unchanged. On the other hand, cocaine did not modify the increase in PTD induced by the increase in frequency of stimulation, but significantly reduced the magnitude of the increase in V max T. The cocaine-induced attenuation of the increase in V max T in response to changes in the frequency of stimulation was abolished by both propranolol and phentolamine. It is concluded that the effect of cocaine on the force-frequency relationship required background activation of α- and β-adrenergic receptors.
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