Differential cleavage rates and sex determination in bovine embryos

The purpose of this study was to determine whether a correlation between embryonic cleavage rates and sex ratios in dairy cattle could be substantiated. Embryos were collected at Day 7 after estrus following superovulation and artificial insemination. Embryos from flushings where at least two different developmental stages within the same flushing could be identified were examined. A total of 476 such embryos was transferred to recipients and 110 calves were born. The sex effect was only demonstrable in flushings yielding embryos of at least three different developmental stages. The sex ratios from this group were 32%, 58% and 62% in the slow, intermediate and fast developing groups of embryos, respectively. The deviations were not significant (P = 0.084). Only 23% of the flushings provided embryos that fell into this cathegory, which limits the use of differential cleavage rates as a sexing method, unless additional embryo culture is introduced to the technique.

Role of Na-K ATPase in regulation of resting membrane potential of cultured rat skeletal myotubes

The role of Na-K ATPase in the determination of resting membrane potential (Em) as a function of extracellular K ion concentration was investigated in cultured rat myotubes. The Em of control myotubes at 37 degrees C varied as a function of (K+)0 with a slope of about 58-60 mV per ten-fold change in (K+)0. Inhibition of the Na-K pump with ouabain or by reduced temperature revealed that this relation consists of two components. One, between (K+)0 of 10 and 100 mM, remains unchanged by alterations in enzyme activity; The second, between (K+)0 of 1 and 10 mM, is related to the amount of Na-K pump activity, the slope decreasing as pump activity decreases. Indeed, with complete inhibition of the Na-K pump, Em does not change over the range of (K+)0 1 to 10 mM. Measurements of 86Rb efflux and input resistance of individual myotubes showed that membrane permeability does not change as (K+)0 increases from 1 to 10 mM but increases as (K+)0 increases further. Monensin, which increases Na ion permeability, increases Em at values of external K+ below 10 mM, and is without effect at higher values of K+ concentration. The effect of monensin is blocked by ouabain. Tetrodotoxin, which blocks voltage-dependent Na+ channels, decreases Em at low (2-10 mM) K+. We conclude that changes in Em as a function of extracellular K+ concentration in the physiological range are not adequately explained by the diffusion potential hypothesis of Em, and that other theories (electrogenic pump, surface-absorption) must be considered.

Some electrophysiological properties of cultured rat cerebral cortical neurons dissociated from fetuses at various gestational ages

Neurons from dissociated cerebral cortex of fetal rat of different gestational ages were grown in culture for up to 4 weeks. Studies of membrane and action potentials, input resistance, neuron size and neurite outgrowth showed that neurons from 7-day fetuses develop rapidly both electrophysiologically and morphologically, but are maintained for only about 2 weeks. In contrast, neurons from 14 to 17 day fetuses mature slowly, but can be maintained for at least 4 weeks. Neurons from both young and old fetuses show an increase in resting Em with age, the maximum value of -60 to -65 mV being attained by about 2 weeks. While neurons can be maintained in serum-free medium, their membrane electrical properties deteriorate with time. Thus, appropriate development of neurons in culture depends upon both the age of the starting tissue, and the presence of adequate, but still undefined factors found in animal serum for at least the first several days in culture.

Dependence of Na+,K+-ATPase and electrogenic component of Em in cultured myotubes on cell fusion

This study was undertaken in order to determine the relation among cell fusion, [3H]ouabain binding and the membrane potential (Em) of cultured rat skeletal muscle. The amount of ouabain bound and the Em both increased with age, the increases being most dramatic following fusion. Inhibition of fusion prevented the developmental increases in both properties of cultured muscle. After fusion, the size of the electrogenic component of Em, determined by the decrease in Em produced by ouabain within 5-10 min, increased independent of the age at which fusion occurred. It is concluded that the increase in Em with age depends on postfusion appearance and activity of Na,K-ATPase.

Hair cell generator potentials

A technique is introduced using a piezoelectric device to stimulate hair cells of a molluscan statocyst while recording their responses intracellularly. Statocyst displacements produced with the technique are calibrated with stroboscopic photography. Properties of the hair cells' response to currents and mechanical stimulation are studied. The hair cell generator potential arises from a conductance increase and, for a certain range, is a logarithmic function of the amplitude of the displacement stimulus.