[Excitatory effect of alpha-adrenoceptor on the transient inward current in sheep myocardium]

Acetyl strophanthidin (AS) 0.05 mumol/L was used to induce a stable model of transient inward current (Iti) of sheep Purkinje fibers. The effect of the alpha-adrenoceptor agonist phenylephrine (PE) 0.3, 1.0 mumol/L on the amplitude and duration of Iti was observed in the presence of propranolol 0.5 mumol/L to block the beta-adrenoceptor. When the preparation was perfused with PE 1.0 mumol/L for 20, 50 min, the amplitude of Iti decreased from control value 12.8 +/- 1.9 nA to respectively 10.7 +/- 1.2 nA (n = 5, P < 0.05) and 9.6 +/- 1.9 nA (n = 5, P < 0.01). The D50 of Iti prolonged correspondingly from control value 145 +/- 24.4 ms to 183.3 +/- 28.1 ms (n = 5, P < 0.05) and 207.5 +/- 34.2 ms (n = 5, P < 0.01). The inhibitory effect of PE on Iti showed a dose- and time-dependent manner. Both the time for arrival of the peak value and for return to baseline of Iti prolonged. This suggests that the kinetics of Iti channel changed in the presence of PE. When the Iti was enhanced by beta- adreceptor agonist isoprenaline (ISO) 1.0 mumol/L, the addition of PE 1.0 mumol/L to the perfusate for 10 min, the inhibitory effect become more pronounced, the amplitude of Iti decreased from 15.6 +/- 3.2 nA to 10.3 +/- 2.2 nA and the D50 of Iti prolonged from 92.5 +/- 14.3 ms to 132.5 +/- 36.0 ms (n = 5, P < 0.01).