CALCIUM ELEVATION IN SHEEP CUMULUS-OOCYTE COMPLEXES AFTER LUTEINIZING-HORMONE STIMULATION

We investigated Ca2+ levels in intact cumulus-oocyte complexes (COCs) on exposure to peak levels of luteinising hormone (LH). Specific prepa rations were used where cumulus corona cells were loaded with a membra ne-permeant Ca2+-sensitive dye (FLUO-3AM), whereas the oocyte was inje cted directly with the nonpermeant form of the dye (FLUO-3). After exp osure to LH, cumulus and corona radiata cells showed distinct rises in intracellular Ca2+ in 50-200 sec. The pattern of Ca2+ response varied in the different cells both for the duration of the transients and fo r their persistence. Interestingly, Ca2+ elevations were recorded in a ll the layers of the cumulus mass, including the innermost layer of co rona cells, demonstrating the wide diffusion of LH receptors. Followin g the Ca2+ raise in somatic cells, an intracellular Ca2+ elevation als o was recorded within the oocyte with a delay of 100-300 sec. The elev ation started at the cortex of the oocyte and then spread all over the ooplasm. The addition of verapamil or manganese chloride did not prev ent LH-induced Ca2+ elevation in the COC, whereas mechanical uncouplin g of cumulus cells from the oocyte prevented any Ca2+ response within the oocyte. The results indicate that cumulus-corona cells are capable of transducing LH message by rising intracellular Ca2+ and show that this signal is rapidly transferred into the oocyte through gap junctio ns. This may result from the direct diffusion of Ca2+ or its putative releaser IP3 from cumulus cells to the oocyte. (C) 1998 Wiley-Liss, In c.