Role of dihydropyridine-sensitive calcium channels in meiosis and fertilization in the bivalve molluscs Ruditapes philippinarum and Crassostrea gigas.
Prophase-arrested oocytes of Ruditapes philippinarum can not be fertilized or stimulated by a depolarizing agent such as an excess of KCl, in contrast to the situation found in Crassostrea gigas. We have performed a comparative study between the two situations found in these species. In vitro, both of these oocytes can be triggered to reinitiate meiosis following a treatment by serotonin which promotes an intracellular calcium surge. Ruditapes and Crassostrea oocytes further arrest in metaphase I, at which stage they can be either activated by sperm or by excess KCl. These treatments trigger an intracellular calcium increase. This suggests that functional voltage-operated Ca2+ channels are expressed in Ruditapes during the course of maturation between prophase and metaphase I. Results obtained using pharmacological tools and direct binding of specific dihydropyridines, strongly suggest that these channels are dihydropyridine-sensitive calcium channels. In Ruditapes they become functional after 5-HT stimulation, their number increasing before GVBD. In Crassostrea the dihydropyridine-sensitive Ca2+ channels are already present at prophase stage and their density is constant from prophase to metaphase I. Moreover, we have shown for Ruditapes and Crassostrea that: 1) the addition of 10 microM of S(-)BayK8644, an agonist of dihydropyridine-sensitive calcium channels to metaphase-arrested oocytes releases them from metaphase block; and 2) incubating these oocytes with nicardipine, a potent blocker of dihydropyridine-sensitive Ca2+ channels, inhibits both their activation by excess KCl or fertilization. Taken together these data suggest that the absence of dihydropyridine-sensitive Ca2+ channels in the membrane of prophase-arrested oocytes of Ruditapes may account for their inability to be fertilized at this stage, while the presence of dihydropyridine-sensitive Ca2+ channels in prophase-arrested oocytes of Crassostrea may explain their fertilizability at this stage.
Leclerc
C
C
Centre de Biologie du Développement, UMR 5547, Université Paul-Sabatier, Toulouse, France.
Guerrier
P
P
Moreau
M
M
D000818:Animals; D049872:Bivalvia; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers; D015220:Calcium Channels / Q000502:physiology*; D004095:Dihydropyridines; D005260:Female; D005306:Fertilization / Q000502:physiology*; D008297:Male; D008540:Meiosis / Q000502:physiology*; D008677:Metaphase; D009865:Oocytes / Q000502:physiology; D011418:Prophase; D012701:Serotonin
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004095:Dihydropyridines; D012701:Serotonin; C038806:1,4-dihydropyridine; D002118:Calcium