A potassium current evoked by growth hormone‐releasing hormone in follicular oocytes of Xenopus laevis.

Abstract

SUMMARY1. Electrophysiological properties of the growth hormone-releasing hormone (GRH) receptor were studied in Xenopus oocytes with an intact follicle cell layer (i.e. follicular oocytes) by measuring whole-cell current using the two-electrode voltageclamp method.2. A slow transient outward current was elicited in oocytes, clamped at -60 mV, by the application of rat GRH but not bovine, porcine, or human GRH.3. The response to GRH was not suppressed by blockers known to inhibit other endogenous receptors present in follicular Xenopus oocytes; blockers used were timolol (2,UM; ,1-adrenergic blocker), theophylline (0-1 mM; purinergic blocker) and atropine (100 nM; muscarinic blocker).4. The current response evoked by rat GRH occurred in a dose-dependent manner. The concentrations of GRH for threshold and maximum responses were 1 and 100 nm respectively and the estimated EC50 (half-maximal effective concentration) was approximately 7 nM. The amplitude and conductance of the response became larger and the latency, time-to-peak and half-decay time were shortened when the concentration of GRH was increased.5. The GRH response was reversibly inhibited by a K+ channel blocker, tetraethylammonium+ (TEA+; 20 mM). The reversal potential for the GRH response was around -100 mV and was compatible with the reported value for a K+ current in Xenopus oocytes. Furthermore, a depolarizing shift of 40 mV in the reversal potential was observed when the external K+ concentration was increased from 2 to 10 mm, agreeing with the Nernst equation. In contrast, no significant shift in the reversal potential was observed by changing the external concentration of Na+ or Cl-.6. The GRH response was not suppressed in oocytes treated with an acetoxy-* Present address:

Mechanism of release of Ca2+ from intracellular stores in response to ionomycin in oocytes of the frog Xenopus laevis.

“…Since responses to chemicals varied from frog to frog (Yoshida & Plant, 1991), oocytes from a single donor were used in the present study to examine the effects of ligands. Also, as the number of observations in the present work was smaller than 30, the t test, one of the parametric tests, was not suitable because it required normal distribution and homogeneity of variance.…”

“…Mature oocytes, at stages V and VI (Dumont, 1972), were collected from the ovary of adult female frogs of Xenopus laevis which were anaesthetized by cooling with ice (Yoshida & Plant, 1991). Oocytes were freed from the connective tissue and follicle cell layer with 2 mg/ml collagenase (Sigma, Type IA) in Ca2+-free saline for 1-5-2-5 h at room temperature (20-23°C).…”

“…The conventional two-electrode voltage-clamp method, using a home-made voltage-clamp system, was used to measure whole-cell currents in Xenopus oocytes as described previously (Yoshida, Plant, Taylor & Eidne, 1989;Yoshida & Plant, 1991). DC resistances of membrane potential-recording and current-injection electrodes, filled with 3 M KCl, were 1-3 MQ and 08-1-5 MQ, respectively.…”

“…The stored data were displayed on a chart recorder (Graphtee SR6335-2L) for illustrations. The reversal potential for current responses to ionomycin was estimated by the ramp method (Yoshida & Plant, 1991); repetitive voltage ramps, 0-8-1 0 s in duration with slopes of 80-120 mV/s, were applied to an oocyte during a response to ionomycin.…”

Growth hormone (GH) and reproduction: a review

“…Application of rat GRF to intact Xenopus follicles elicits an oocyte membrane transient outward current. The response involves the follicule cell layers, appears dependent on activation of K + channels and involves a cAMP pathway (Yoshida and Plant 1991).…”