Calcium-dependent, iproveratril-resistant contractions of aortic strips induced by a convulsant barbiturate

At 3 X 10(-5) M concentratons, 5-(2-cyclohexylideneëthyl)-5-ethyl barbituric acid (CHEB) causes a contraction of rabbit aortic strips that is equivalent to about half of the maximal response to norepinephrine. If a calcium-free medium is substituted for normal bathing solution and drug is added immediately thereafter, there is no response to the convulsant barbiturate but a nearly normal response to norepinephrine. When calcium is replaced in stepwise increments, the contractor response to CHEB appears in corresponding increments. The prompt influence of changes in external calcium-concentration suggests that CHEB-induced contraction occurs as a result of calcium influx into cells rather than a release of free calcium from previously accumulated intracellular stores. CHEB effect, like that of potassium, is prevented by pretreatment with lanthanum (5 X 10(-3)M). CHEB differs from potassium in that its effects are even more sensitive to changes in extracellular calcium concentration. Nevertheless, CHEB can contract aortic strips in the presence of iproveratril at a concentration (10(-4)M) that blocks all response to potassium (100 mM).