Effects of SCN substitution for Cl- on tension, [Ca2+]i, and ionic currents in vascular smooth muscle

Substitution of thiocyanate ions (SCN-) for chloride ions (Cl-) in the extracellular medium of aortic rings and strips causes a biphasic contractile response; initial relaxation followed by sustained contraction. 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 within 1-2 min, subsequently returned to original levels within 4-5 min, and then increased to a higher plateau over the next 10 minutes. This pattern of change in Ca2+]i is identical to the pattern of tension changes in aortic rings, but it occurs somewhat faster. Partial substitution of SCN- for Cl- elicited increased, but no preceding decrease in Ca2+]i. In the absence of external Ca2+, anion substitution elicited the decrease in Ca2+]i but not the subsequent increase. Verapamil (1 microM) blocked the increased Ca2+]i phase but not the decreased Ca2+]i phase; whereas, R+ verapamil (up to 5 microM for 20 min), an inactive enantiomer, caused no change. Ionic current measurements obtained using whole cell patch and current clamp techniques revealed two responses to anion substitution: (a) a rapid, transient outward shift in holding current, and (b) a sustained increase in peak current and a hyperpolarizing shift in voltage sensitivity of Ca2+ channels. The calcium channel blocker PN200-110 blocked SCN(-)-enhanced current but had no effect on the changes in holding current. S- verapamil, but not R+ verapamil, reduced SCN(-)-enhanced current. In current clamp mode, SCN- caused an initial hyperpolarization followed by a slow depolarization punctuated by spikes. Thus, SCN- causes changes in vascular smooth muscle Ca2+]i that could underlie both phases of its effects on tension in isolated aortas and may be explained by the following model: an initial outward shift in current causes hyperpolarization with a consequent decrease in cell excitability, and the somewhat slower increase in Ca2+ channel excitability eventually leads to enhanced calcium influx and tension. These data shed light on possible mechanisms underlying gender-related differences in VSMC physiology.