Features of a calcium independent, caffeine sensitive outward current in single smooth muscle cells from guinea pig protal vein.

Using the single electrode voltage-clamp technique, vascular smooth muscle cells from the guinea pig portal vein showed an initial inward (Ca2+) current followed by an outward current which peaked within 100 ms and then declined to a steady level in a few seconds. Caffeine (1 mmol/l) selectively blocked the transient component of the outward current (ITO) and allowed differentiation of the outward current into two components: ITO and a caffeine resistant background current. The potassium channel blockers TEA (10 mmol/l) and 4-AP (5 mmol/l) produced about 90% suppression of ITO. ITO was identified as a calcium independent potassium current. Analysis of the inactivation and recovery from inactivation of ITO revealed similarities to the A-current first described for molluscan neurones (11) and later for crista terminalis of rabbit heart (4). Being much slower than this current it also bears similarities in its inactivation kinetics to a transient outward current identified in rabbit portal vein (2, 6).