Regulation of beta-adrenergic response in Xenopus laevis oocytes.

It has been demonstrated that Xenopus laevis oocytes are sensitive to catechol­ amines. The oocyte response is characterized as being of beta adrenergic nature and is electrophysiologically measured as a transient increase in potassium con­ ductance (hyperpolarizing current in holding potential —40 mV) (VanRenterghem et al.1985).The aim of this paper was to study the decline of the oocyte response observed during continuous isoprenaline application. The experiments were performed on oocytes from the ovary of Xenopus laevis prepared manually. All experiments were carried out with oocytes at stages V and VI (Dumont 1972).The oocytes were stored in a modified sterile Barth's medium at 15°C (Stinnakre and VanRenterghem 1986). The two-microelectrodes voltage clamp method was used for electrophysiological recordings. The membrane poten­ tial of the oocytes was clamped at — 40mV during all experiments. One oocyte per experiment was placed into a chamber continuously perfused with Ringer saline at 3ml/min. Drug solutions were applied by bath perfusion after adjusting pH to 7.2-7.4, and/or by pressure injection into the cell through a third micropipette with a tip diameter less than 10 /xm. The oocyte response to isoprenaline is characterized by a latency of the re­ sponse (approximately 30 s) followed by a fast increase of the outward current (Fig. la). This current is the result of a change in potassium permeability (Van­ Renterghem et al. 1985). The time to peak was reached within 100 s. With isoprenaline concentrations exceeding 10 _7 mol/l, the time to peak was shorter. The current decreased in the continuous presence of the drug. The effect of isoprenaline on Xenopus laevis oocyte is mediated by beta adren­ ergic receptor, adenyl cyclase system and cAMP-dependent changes of potassium conductance (VanRenterghem et al. 1985). The decline in K + permeability after a transient increase during continuous isoprenaline treatment may be due to: 1) fast inactivation of K + ion channels,