Membrane responses of smooth muscle cells in guinea pig cochlea spiral modiolar artery to norepinephrine and acetylcholine

The inner ear blood circulation plays an important role in many physiological and pathological conditions of hearing function. Heterogeneity in physiological and pharmacological membrane properties of the vascular smooth muscle cells has been shown among various blood vessel beds in different organs. The cellular properties of the smooth muscle in the inner ear vascular system are poorly understood. Using acutely isolated spiral modiolar artery superfused with a 36°C physiological (Krebs) solution containing 5 mM potassium, intracellular recordings were made with microelectrodes filled with 2 M KCl. The resting membrane potential was -78.1 ± 3.4 mV (mean ± SE, n=31), ranged from -68 to -87 mV, which was concentration-dependently depolarized by high K⁺(10 - 50 mM) solutions to about the potassium equilibrium potentials. Ba²⁺ (0.5 mM) depolarized the cells to -34 ± 3 mV in all cells tested (n=10). In normal Krebs, norepinephrine (NE, 1-100 μM) and acetylcholine (ACh, 1-100 μM) induced ususally a small depolarization or no membrane potential change. In presence of 0.5 mM Ba²⁺, NE consistently caused a depolarization which was blocked by 2 μM of prazosin, idazoxan but not yohimbine, whereas ACh induced a robust hyperpolarization, which was completely blocked by 0.1 μM atropine or 50 nM 4-DAMP methiodide. Our data suggest that the smooth muscle cells in the artery express abundant inward rectifying potassium channels and the vascular cells express functional α and M₃ muscarinic receptors.