Three membrane-conductance mechanisms activated by noradrenaline in vascular smooth muscle.

Membrane mechanisms activated by noradrenaline in vascular smooth muscle have been studied with the perforated patch technique. From ion substitution experiments it was evident that the application of noradrenaline increased the membrane permeability to potassium, chloride and cations (non-selective) through three distinct ion channels. The K and Cl currents were activated when the intracellular calcium concentration (Cai) was increased and are therefore calcium-activated mechanisms (IK(Ca) and ICl(Ca)). In contrast, the non-selective cation current (Icat) was not activated directly by an increase in Cai, but calcium appeared to possess a permissive role in the generation of Icat. The non-selective cation conductance was highly permeable to divalent cations, but was not inhibited by calcium-antagonists. IK(Ca), ICl(Ca) and Icat were blocked by the alpha 1-adrenoceptor selective antagonist prazosin (10(-8) M), but were unaffected by yohimbine (10(-6) M) and, thus, all three membrane mechanisms are linked to the alpha 1-adrenoceptor.