Induction of a sodium-dependent depolarization by external calcium removal in human sperm.
Removal of external calcium with EGTA (from 2.5 mm to nanomolar levels) caused a remarkable depolarization in human sperm. This depolarization was initially fast. It was followed by a slow phase that brought the Vm to values of over 0 mV in 1-2 min. The slow and sustained phase correlated with a sustained decrease in intracellular calcium. However, calcium removal still induced depolarization in sperm with enhanced intracellular calcium (induced by progesterone), indicating that the sustained depolarization was not caused by a sustained intracellular calcium decrease. The depolarization was reduced as the external sodium content was substituted with choline, indicating that it was due to a sodium current, and was observed in lithium but not in tetramethylammonium-containing medium. In low sodium medium, the addition of sodium after calcium removal induced depolarization to the extent of which slightly increased in 2 min. The depolarization was completely inhibited by external magnesium (Ki = 1.16 mm). The addition of calcium or magnesium to calcium removal-induced depolarized sperm induced hyperpolarization that was inhibited by ouabain and was also prevented in medium without potassium, suggesting that the activity of the electrogenic Na+,K+-ATPase was involved. The conductance activated by calcium removal might unveil the presence of a calcium channel that in the absence of external calcium allows sodium permeation and that in normal conditions might contribute to the resting intracellular calcium concentration.
González-Martínez
Marco T
MT
Departamento de Farmacología, Facultad de Medicina, Universidad Nacional Autónoma de México, Ciudad Universitaria, CP 04510 Apartado Postal 70-297 México, D. F., México. tuliog@servidor.unam.mx
D000293:Adolescent; D000328:Adult; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002138:Calibration; D002614:Chelating Agents / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004533:Egtazic Acid / Q000494:pharmacology; D004594:Electrophysiology; D005731:Ganglionic Stimulants / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology; D008274:Magnesium / Q000737:chemistry / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials; D011374:Progesterone / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013094:Spermatozoa / Q000378:metabolism*
D002614:Chelating Agents; D005731:Ganglionic Stimulants; D000644:Quaternary Ammonium Compounds; D011374:Progesterone; D004533:Egtazic Acid; D008094:Lithium; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; C027917:tetramethylammonium; D008274:Magnesium; D002118:Calcium