Changes in plasma membrane ionic permeability and related contractile responses in vascular smooth muscle at hypoxia

Abstract

Present study was designed to examine the effects of low P O 2 (9.1-3.9 kPa) on Ca 2+ and K + permeability in plasma membrane and related contractile responses of vascular smooth muscle (VSM). Recordings of VSM contractile force were done by the force displacement transducer coupled to a polygraph. Cellular 45 Ca uptake was determined by the lanthanum method. Ionic current measurements were made using patch-clamp technique (whole cell configuration). It was shown that the gradual decrease in bath P O 2 below 14.3-13 kPa as a rule resulted in the inhibition of VSM myogenic activity (portal vein) and the relaxation of VSM preconstricted with noradrenaline and high-K + solution or transmural electrical stimulation (aorta and portal vein). In muscles from aorta, hypoxia often led to biphasic response: a pronounced dilation following phasic (transient) contraction; in portal vein hypoxia induced the dilation only. The constrictor component of aorta response to hypoxia was endothelium-dependent, but dilations in both cases were independent of intact endothelium. The similar hypoxia-like effects in VSM were produced by a Ca 2+ -free solution and Ca 2+ channel blockers. 45 Ca uptake was significantly decreased in muscles from portal vein at hypoxia. The lowering of bath P O 2 led to decrease of peak amplitude of Ca 2+ current ( I Ca ) in both kinds of VSM cells. Hypoxia also increased the transient [ K(Ca)t ] and oscillatory [ I K(Ca)o ] components of Ca 2+ -activated K + current [ I K(Ca) ] and was able to increase the amplitude and frequency of spontaneous transient outward currents (STOCs) in freshly dispersed single cells from portal vein. Intracellular ‘injection’ of phosphocreatine (PCr), but not adenosine triphosphate (ATP), to VSM cell myoplasm using positively charged liposomes prevented the inhibitor effects of low P O 2 on VSM contractile activity. The effects of hypoxia were completely reversible as evident from contractile and electrophysiological measurements. Thus, low P O 2 (hypoxia) may affect Ca 2+ permeability and hence the VSM contractility for at least two mechanisms: (i) by the decrease of a number of open Ca 2+ channels as a result of the damage in their phosphorylation; (ii) by Ca 2+ channel inactivation resultant from a rise in intracellular Ca 2+ which could be induced by a failure of active Ca 2+ outward transport or/and intracellular Ca 2+ sequestration.