Facial vein of the rabbit. Intracellularly recorded hyperpolarization of smooth muscle cells induced by beta-adrenergic receptor stimulation.

A segment of the facial vein of the rabbit gains myogenic tone when placed under tension in a tissue bath (Winquist and Bevan, 1977); low frequency electrical stimulation of intramural nerves, or application of norepinephrine or isoproterenol produces relaxation, which is blocked by propranolol (1.0 microM). We measured membrane potential of single smooth muscle cells with microelectrodes during this beta-adrenergic receptor-mediated relaxation of the vein segment. Hyperpolarization (up to 30 mV) invariably accompanied relaxation. Average maximum resting potential recorded in normal Krebs solution was -46.8 +/- 3.6 mV (mean +/- SE); this increased to -70.8 +/- 3.0 mV when high concentrations (greater than 10 nM) of isoproterenol were added to the bath. Tone and membrane potential were very sensitive to intramural nerve stimulation. At 3-4 Hz, relaxation was about half maximal and hyperpolarization was 6-9 mV. The maximum hyperpolarization to nerve stimulation was 20-30 mV, about the same magnitude as that produced by isoproterenol. Hyperpolarization clearly preceded relaxation by about 0.2 second. The magnitude of hyperpolarization in facial vein is much larger (2-10 times or more) than that recorded in other tissues during inhibition produced by stimulation of beta-adrenergic receptors. The large hyperpolarization may augment the effectiveness of inhibition (by curtailing potential dependent influx of Ca++o, for example) in this tissue compared to that in other tissues in which hyperpolarization is slight or absent. The ionic mechanism underlying hyperpolarization resulting from stimulation of beta-adrenergic receptors remains unclear.