Calcium pools in sea urchin eggs: Roles of endoplasmic reticulum and mitochondria in relation to fertilization

Abstract

A preparation of sea urchin eggs permeabilized with digitonin (40 μM for 2.5 min) was used to study the kinetic characteristics of the two cellular compartments suspected to play a key role in cellular calcium transfer during fertilization: an ATP-dependent Ca 2+ pool (Km = 0.47 μM; Vm = 0.48 nmol/min.mg protein) probably located in the endoplasmic reticulum and a mitochondrial Ca 2+ pool (Km = 1.50 μM; Vm = 0.12 nmol/min.mg protein). Fertilization triggered a decrease in the rate of ATP dependent uptake by the non-mitochondrial pool (Km = 0.59 μM; Vm = 0.15 nmol/min.mg protein) while it transiently increased the Ca 2+ uptake into mitochondria (2 min post-fertilization: Km = 2.20 μM; Vm = 0.40 nmol/min.mg protein). Microanalysis studies performed on quickly frozen, freeze substituted and embedded eggs showed a transient Ca 2+ enrichment of mitochondria soon after fertilization thus suggesting that mitochondria behave as a Ca 2+ sink at fertilization. Results are discussed in relation to the role of endoplasmic reticulum and mitochondria in handling free calcium during the early period following sea urchin egg fertilization.