Local Regulation of Blood Flow

Abstract

In vascular smooth muscle as in other excitable structures the K⁺ permeability is considerably higher than the Na⁺ permeability (P_K: P_Na = 1: O.024) (Siegel and Schneider, 1981). Therefore, the intra- and extracellular K⁺ ion distribution play a decisive role in the passive potential genesis. Electromechanical coupling provided, a change of internal and/or external K⁺ concentration can thus influence vascular tone. With the aid of spectroscopic and morphometric data the K⁺ fraction located in the intracellular space of smooth muscle cells is calculated to be 6/7 of the total potassium in a vessel wall, that located in the extracellular space to be 1/7. 43% of the extracellular potassium is distributed in the interstitial fluid space, while 57% is bound to connective tissue structures. The microdynamic K⁺ binding properties of the latter fraction permit, under pH or concentration shifts of other cation species, a K⁺ release from or adsorption to the polyanionic macromolecules of vascular connective tissue, which can considerably alter the external K⁺ concentration close to the cell membrane (Siegel et al., 1977a). Further, it is known from studies with nuclear magnetic resonance spectroscopy that a shift in proton concentration not only changes the binding properties of cations to polyelectrolytes but also directly alters their conformation (Gustavsson et al., 1981). Thus, besides the effect of K⁺ ions on vascular smooth muscle cells, we have also studied the influence of the external pH value on membrane permeability.