Effects of ionophores A23187 and X537A on vascular smooth muscle activity

The ionophore A23187 initiated contractions in both dog coronary artery and rabbit aortic strips in a dose-dependent manner whereas X537A contracted rabbit aortic strips and relaxed KCl-induced contractions in dog coronary artery. Diphenylhydantoin (DPH) and verapamil ccompletely abolished KCl-induced responses in both vascular tissues. DPH (10(-4) M) partially prevented A23187-induced responses in both tissues and verapamil (33 micrometer) partially prevented A23187-induced responses in dog coronary artery but not in rabbit aorta. Phenoxybenzamine (10(-5) M), but not practolol, significantly reduced X537A-induced contractions in rabbit aortic strips but did not affect A23187-induced contractions in either tissue. The inability of DPH and verapamil to completely block the A23187 contractions leads one to conclude that these agents do not completely block calcium influx, or that A23187 does not work solely by increasing the permeability of the cell membrane to calcium. The effect of X537A on rabbit aorta, however, may be mediated at least partially, via release of catecholamines.