EFFECTS OF SCN- SUBSTITUTION FOR CL- ON TENSION, [CA2-MUSCLE(](I), AND IONIC CURRENTS IN VASCULAR SMOOTH)

Substitution of thiocyanate ions (SCN-) for chloride ions (Cl-) in the extracellular medium of aortic rings and strips causes a biphasic con tractile response; initial relaxation followed by sustained contractio n. Alterations in these responses are sex-specific, and may elucidate fundamental differences in vascular function between males and females . In order to investigate the role of changes in intracellular Ca2+ ([ Ca2+](i)) in these changes in tension, we investigated effects of SCN- on [Ca2+](i) and ionic currents in vascular smooth muscle cells (VSMC ). Extracellular substitution of SCN- for Cl- caused a biphasic change in [Ca2+](i). Initially, [Ca2+](i) decreased, reaching a minimum with in 1-2 min, subsequently returned to original levels within 4-5 min, a nd then increased to a higher plateau over the next 10 minutes. This p attern of change in [Ca2+](i) is identical to the pattern of tension c hanges in aortic rings, but it occurs somewhat faster. Partial substit ution of SCN- for Cl- elicited increased, but no preceding decrease in [Ca2+](i). In the absence of external Ca-i(2+) anion substitution eli cited the decrease in [Ca2+](i) but not the subsequent increase. Verap amil (1 mu M) blocked the increased [Ca2+](i) phase but not the decrea sed [Ca2+](i) phase; whereas, R+ verapamil (up to 5 mu M for 20 min), an inactive enantiomer, caused no change. Ionic current measurements o btained using whole cell patch and current clamp techniques revealed t wo responses to anion substitution: (a) a rapid, transient outward shi ft in holding current, and (b) a sustained increase in peak current an d a hyperpolarizing shift in voltage sensitivity of Ca2+ channels. The calcium channel blocker PN200-110 blocked SCN--enhanced current but h ad no effect on the changes in holding current. S- verapamil, but not R+ verapamil, reduced SCN--enhanced current. In current clamp mode, SC N- caused an initial hyperpolarization followed by a slow depolarizati on punctuated by spikes. Thus, SCN- causes changes in vascular smooth muscle [Ca2+](i) that could underlie both phases of its effects on ten sion in isolated aortas and may be explained by the following model: a n initial outward shift in current causes hyperpolarization with a con sequent decrease in cell excitability, and the somewhat slower increas e in Ca2+ channel excitability eventually leads to enhanced calcium in flux and tension. These data shed light on possible mechanisms underly ing gender-related differences in VSMC physiology.