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.
Standley
P R
PR
Department of Physiology, Wayne State University School of Medicine, Detroit, MI, USA.
Zhang
F
F
Ravi
J
J
Ram
J L
JL
Sowers
J R
JR
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001013:Aorta, Thoracic; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002712:Chlorides / Q000494:pharmacology*; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D013861:Thiocyanates / Q000494:pharmacology*; D013997:Time Factors; D014700:Verapamil / Q000494:pharmacology
D002712:Chlorides; D013861:Thiocyanates; D001464:Barium; D014700:Verapamil; D000109:Acetylcholine; D002118:Calcium