Changes in ion permeability and membrane potentials during early echinoderm development: electrophysiological and tracer-flux determinations

Effect of inositol and glycine with increasing sodium chloride and constant osmolality on development of rabbit embryos

PURPOSE

Many commercially available culture media have a high sodium content as sodium chloride and sodium bicarbonate. We examined the effects of osmolytes, inositol and glycine, on embryos cultured in synthetic RD medium with a lower concentration of NaCl (93 mM) and a higher concentration of NaCl (116 mM) with media held constant at 270 mosmols.

RESULTS

There were no significant effects of either glycine or inositol on embryo growth when the embryos were cultured in RD medium with 93 mM NaCl (P > 0.05). Culture of one-cell embryos for 72 h in RD medium with 93 mM NaCl, 116 mM NaCl and 116 mM NaCl containing 0.56 mM inositol resulted in 77%, 14% and 55% expanded blastocysts, respectively (P < 0.05). Corresponding values for two-cell embryos cultured for 67 h were 84%, 34% and 66% expanded blastocysts (P < 0.05). When one-cell and two-cell embryos were cultured in RD medium with 93 mM NaCl, 116 mM NaCl, 116 mM NaCl plus 1 mM glycine and 116 mM NaCl plus 1 mM glycine and 0.56 mM inositol, expanded blastocysts from one-cell embryos were 79%, 19%, 49%, and 48%, respectively (P < 0.05), and expanded blastocysts from two-cell embryos were 91%, 32%, 52%, and 53%, respectively (P < 0.05).

CONCLUSION

Inositol and glycine presumably behave as osmolytes in providing substantial protection for rabbit one-cell and two-cell embryos cultured in a medium with high NaCl concentration.

Electrical maturation of the murine oocyte: an increase in calcium current coincides with acquisition of meiotic competence

We have used the whole-cell recording technique to compare three stages of primary and secondary oocytes from F1 hybrid mice (C57BL/6J x SJL/J): neonatal germinal vesicle (NGV) stage primary oocytes from 10- to 20-day-old, prepubescent mice; mature germinal vesicle (MGV) stage primary oocytes from 12-week-old, post-pubescent, superovulated mice; first polar body (FPB) stage secondary oocytes from 12-week-old, post-pubescent mice during the normal oestrus cycle or following superovulation. NGV, MGV and FPB oocytes all exhibit two voltage-dependent currents: an inward, rapidly activating/inactivating current, and an outward, slowly activating/non-inactivating current. In 1.5 mmol/l external Ca the average peak inward current is -2.9, -12.4 and -13.8 microA/cm2 in NGV, MGV and FPB oocytes, respectively. In 20 mmol/l Ca these currents increase and the reversal potential shifts to the right. The outward current decreases slightly with growth and development: at 40 mV test potentials, NGV oocytes have average outward currents of 8.9 microA/cm2, and MGV and FPB oocytes have currents of 5.0 and 5.5 microA/cm2, respectively. Thus, MGV oocytes express FPB current patterns. The reversal potentials, kinetics and pharmacology of the currents indicate that Ca channels carry the inward current and K channels carry the outward current. During growth in vivo a gradual depolarisation accompanies maturation. Resting potentials ranged from -45 to -30 mV in NGV oocytes to -35 to -17 mV in MGV oocytes to -20 mV to -3 mV in FPB oocytes. These data suggest that a selective increase occurs in the number of Ca channels during oocyte growth. This increase precedes nuclear maturation and coincides with the acquisition of meiotic competence.

Changes in permeability of sea urchin egg membrane to urea after fertilization or activation

1. Fertilization or artificial activation of the sea urchin egg, are known to increase the plasma membrane permeability to ions and small metabolites. We have examined the permeability properties of the egg membrane to urea. 2. Urea influx is not saturable and is slightly dependent on temperature. It is inhibited by phloretin. 3. Fertilization leads to a biphasic increase in urea flux kinetics. The first acceleration lasts about 10 min. The second acceleration is maintained at least up to the first cleavage. 4. The ionophore A 23187 mimics exactly fertilization. 5. Weak bases such as procaine lead to a gradual acceleration in urea flux kinetics. The response depends on the dose of procaine added. 6. The inhibitor of oxydative phosphorylation 2,4-DNP fails to block the accelerations triggered by fertilization or activation by A 23187. 7. We discuss the results in light of the early and late events of egg activation.

Electrophysiology of differentiating mouse spermatozoa

The morphological aspects of spermatogenesis are well described in many mammalian species, but functional changes are not completely understood. Electrophysiological parameters were investigated in primary spermatocytes and early and late spermatids isolated from the seminiferous tubules of the mouse. Substantial changes were not detected in membrane potential between different developmental stages. Membrane potential was dependent on both potassium and sodium ion concentration gradients, but not on chloride gradients. The ratio of the permeabilities PNa/Pk varied according to the extracellular concentrations of sodium and potassium. Ouabain, a specific inhibitor of Na+, K+-activated ATPase, produced a maximal reduction in membrane potential of 20%. Comparisons were drawn between differentiating germ cells and previously determined properties of mature spermatozoa.

Chemical messengers in development: a hypothesis

The hypothesis that physiological and developmental regulatory mechanisms are similar has been presented. Well-known developmental systems chosen illustrate the capability of the model to suggest a simple mechanism underlying the effects on development of a diverse group of chemicals. This hypothesis might be applied to other systems including the induction of the lens, limb regeneration, and the induction of the head of hydra (124). I have proposed this hypothesis not only because it permits consideration of a complex and varied array of experimental observations as reflections of a simple basic biochemical mechanism, but because recent technical advances in instrumentation and methods allow it to be directly tested. The fluorescent antibody method for the cytochemical measurement of cyclic nucleotides provides a means for investigating changes in the concentrations of cyclic nucleotides in developing cells and could also be used to detect neurotransmitters in developing cells. Similarly, the scanning electron microscope in the emitted x-ray mode provides a method for measuring changes in the content and distribution of cations within developing cells. The hypothesis presented here suggests pleasing asceticism on the part of eukaryotes. It suggests that simple derivatives of metabolites, including neurotransmitters and cyclic nucleotides, are linked together as regulatory molecules throughout the eukaryotes. The neurotransmitters are suggested to have a more general role in information transmission in eukaryotes than is generally accepted. They are hypothesized to have progressed during evolution from being intracellular messengers to a role as intercellular messengers for the relatively slow communication of developmental informatbn; and, finally, this process has culminated with their participation in the rapid intercellular communication mediated by nerves. The thought that the complex pictures of physiological regulation and of the construction of a complex multicellular organism like man might be painted with so few colors is quite satisfying.