| Popis: |
Vincent et al. (1) reported that injections of EGTA giving final cytosolic concentrations of about 8 mM had no detectable effects on cytokinesis; but final concentrations of about 30 n-f the true figure seems close to a quarter (2, 3)]. Inhibition at such extremely high buffer concentrations might well suggest that calcium is not involved in the natural control of cytokinesis in Xenopus eggs. However, the results of our study of buffer inhibition in fucoid eggs (4) suggested a reinvestigation with another buffer more closely matched to the calcium level in the cytosolic region responsible for initiating and driving the division process (5). Table I shows the results of injecting 5,5’-dibromo-BAPTA, a buffer with a cytosolic Kn of about 5 pM. The lower figure in each range corrects for total accessible volume, on the assumption that about 20% of the egg volume is accessible to the injected buffer; buffer accessibility was calculated, in turn by taking 70% of the egg volume as inaccessible yolk (2) and about 70% of the cytosol as water (3). The higher figure in each range takes into account the likelihood that, at the time of action, the buffer had diffused out through a sphere only about 800 pm in diameter instead of filling the whole 1200 pm diameter egg. This buffer is about one hundred times more effective than EGTA in delaying or blocking cytokinesis. Detectable delays occur in response to final cytosolic buffer concentrations as low as 0.1 mM. This finding supports both the hypothesis that calcium controls cytokinesis, and the validity of the following equation (4) in Xenopus eggs |
