Free extracellular [Ca2+] at photoreceptor level equals that in vitreous in frog and carp eyes

The extracellular Ca2+ concentration has a strong influence on the function of retinal photoreceptors. In the present study both single- and double-barrelled Ca2+-sensitive microelectrodes were used to measure the free calcium ion concentration in the extracellular fluid bathing the photoreceptors in opened excised eyes of frogs and carps. The electrode tip was localized electrophysiologically by voltage drops related to current pulses passed radially through the preparation, using the high-resistance pigment epithelium as a landmark. The measurements showed that the extracellular Ca2+ concentration is very close or identical to that prevailing in the vitreous humour, 1.0-1.1 mM in the frog retina and 1.2-1.4 mM in the carp.