Intracellular free Ca2+ changes during physiological polyspermy in amphibian eggs

Abstract

We have made the first measurements of intracellular free calcium activity ([Ca2+]i) in urodele eggs during physiological polyspermic fertilization. Jellied eggs of the urodele amphibian Pleurodeles waltlii were impaled with intracellular Ca(2+)-selective microelectrodes and inseminated under various conditions of sperm:egg ratio to obtain various degrees of polyspermy. In 17 out of 45 cases the egg [Ca2+]i level (0.41 microM) showed no variation following fertilization. In 28 other cases, however, the egg displayed a slow increase in [Ca2+]i of 0.15 microM, starting around 15 minutes after fertilization and reaching a plateau level around 10 minutes later. The amplitude of the fertilization-associated increase in [Ca2+]i was found to be independent of the number of sperm interacting with the egg surface. Measurements with two Ca(2+)-microelectrodes impaled in single eggs showed that the increase in [Ca2+]i did not simultaneously occur at distinct places within the egg cortex and, in some cases, could be detected by only one of the two microelectrodes. This latter observation, as well as the absence of [Ca2+]i change at fertilization in some experiments, strongly suggested that each sperm interacting with the egg might, at various places, trigger a localized, non-propagating change in [Ca2+]i. Experiments in which eggs were locally inseminated, using a micropipette directed towards the site of impalement of one of the two Ca(2+)-microelectrodes, clearly established that [Ca2+]i changes, although incapable of propagating over the entire egg cortex, might nevertheless travel very slowly over short distances, their amplitude vanishing rapidly as they propagate from around the sites of sperm entry.(ABSTRACT TRUNCATED AT 250 WORDS)