ENDOGENOUS TRYPSIN RECEPTORS IN XENOPUS OOCYTES - LINKAGE TO INTERNALCALCIUM STORES

The effects of the protease trypsin, externally applied to full-grown oocytes of Xenopus laevis, were studied using electrophysiology and fl uorometry. The following results were obtained: trypsin in concentrati ons of 0.1 mu g/ml to 1 mg/ml liberated Ca2+ from internal stores and evoked large transient currents of up to 5 mu A in bath solutions cont aining 1 mM or no Ca2+. The response desensitized for 50 minutes and r ecovered at longer times. Transient currents could also be elicited by tryptic impurities in commercially available collagenase used for def olliculation of oocytes. Application of chymotrypsin (0.01 or 1 mg/ml) or of thrombin (3.4 ng/ml or 0.34 mg/ml) neither evoked currents nor desensitized trypsin responses. Incubation with 1 mu g/ml Pertussis to xin for 20 to 25 hours prevented the Ca2+ release from internal stores and the activation of transient currents by trypsin. We propose that endogenous receptors in the oolemma, specific for trypsin, are linked to internal Ca2+ stores via Pertussis toxin-sensitive G proteins. Thus , receptor activation by external trypsin raises internal Ca2+ and the reby opens Ca2+-activated Cl- channels in the oolemma.