H⁺ Gradient Changes: Their Measurement and Their Significance in Cell Stimulation

Abstract

As the sequence of events accompanying cell stimulation and oocyte fertilization becomes better known it has become clear that changes in the membrane potential as well as those in intracellular pH and pCa⁺⁺ play an important role (1–21)Table I. Some of the possible ion gradient changes in the response of any cell possessing a receptor R for a given specific stimulus S can be depicted pictorially as in Figure 1. Although recognition and binding of the stimulus to its receptor are clearly the initiating steps, the subsequent sequence is not yet clear. Depolarization can be and in some cells has been (19–21) demonstrated to involve an influx of Na⁺ ions. While there is some evidence that an accompanying H⁺ outflow may be attributable to the stimulus induced opening of an Na⁺-H⁺ antiport as depicted in Figure 1 (13,19), the temporal resolution has not been sufficiently great to allow one to conclude that the Na⁺ and H⁺ flux changes are simultaneous rather than sequential. A Na+ influx undoubtedly stimulates a Na⁺-K⁺ ATPase in many types of cells (22), as well as a release of Ca⁺⁺ from the membrane into the cytoplasm (23). Whether these changes in intracellular cation concentrations trigger membrane-bound enzymes (e.g. the lipases initiating the prostaglandin synthesis pathway in platelets (24)), enhance metabolic activity (25), act as signals for the fusion of organelles with the membrane or have other effects as “secondary” messengers remains the object of a number of studies.