Different electrical responses to vasoactive agonists in morphologically distinct smooth muscle cell types

Abstract

Vascular smooth muscle cells (SMCs) in the blood vessel wall are frequently heterogeneous in nature, differing in their gross morphology, size, and shape, subcellular organelles, cytoskeleton, and contractile protein composition. In adult rat arterial vessels, two populations of SMCs have been shown to predominate: elongated bipolar cells, representing the majority of cells, and epithelial-like SMCs. We examined the ionic responses of these two types of SMCs, isolated by multiple subculture, to vasoactive stimuli. Elevations in intracellular Na+ and Ca2+ were measured with SBFI and fura 2, respectively, and changes in membrane potential were measured using the potential-sensitive fluorescent probe bis-oxonol. The resting membrane potential of the elongated bipolar cells was less negative than that of the epithelial-like SMCs. Exposure of the elongated SMCs to endothelin 1, α-thrombin, or arginine vasopressin induced elevations in [Ca2+](i) and [Na+](i) and membrane depolarization. Depolarization occurred because of entry of both Na+ and Ca2+, and pharmacological blockade of Cl- or K+ channels did not attenuate the depolarization. In contrast, when [Ca2+](i) was elevated by the same agonists in the epithelial-like SMCs there was a pronounced hyperpolarization that appeared to be the consequence of enhanced activity of charybdotoxin-sensitive Ca2+-activated K+ channels because it was abolished by charybdotoxin (20 nmol/L), partially attenuated by tetraethylammonium chloride (10 mmol/L), and unaffected by apamin (1 μmol/L), glibenclamide (1 μmol/L), or 4-aminopyridine (5 mmol/L). Chelation of [Ca2+](i) also abolished the hyperpolarization; instead, a small depolarization was observed. We conclude that SMCs that exhibit different morphological characteristics can also differ with respect to the ability of vasoactive agonists to activate Ca2+-activated K+ channels. It is suggested that redistributions in SMC populations in proliferative vascular diseases may alter the responsiveness of vessels to vasoactive stimuli via cell type-specific modulation of specific membrane ion channels.