Delayed afterdepolarizations and triggered activity induced in feline Purkinje fibers by alpha-adrenergic stimulation in the presence of elevated calcium levels.

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Abstract
We studied the ability of alpha-adrenergic stimulation to induce delayed afterdepolarizations and triggered activity in Purkinje fibers from cat hearts in the presence of an elevated Ca++ concentration. Delayed afterdepolarizations could not be induced at drive cycle lengths of 200 to 500 msec in the presence of extracellular Ca++ concentrations of 2.7 to 8.1 mM. However, the addition of 10(-5)M phenylephrine in the presence of 5 X 10(-7)M propranolol elicited delayed afterdepolarizations in eight of 10 preparations at a Ca++ concentration of 8.1 mM; nondrive-triggered action potentials were recorded from three of the preparations. These afterpotentials were completely suppressed by 5 X 10(-7)M prazosin or 10(-6)M phentolamine. In the presence of 5 X 10(-7)M propranolol, 10(-5)M phenylephrine prolonged action potential duration and this effect was suppressed by 5 X 10(-7)M prazosin. Methoxamine, at a concentration of 5 X 10(-6)M, was also observed to potentiate delayed afterdepolarizations in all of three preparations studied. These results demonstrate that alpha-adrenergic stimulation can induce afterpotentials in the presence of elevated Ca++ levels in cat hearts. Stimulation of alpha-adrenoceptors may be responsible for arrhythmias under Ca++-loaded conditions such as ischemia and coronary reperfusion.

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