The intracellular arsenazo signal indicating the transient light-evoked change in cytosolic Ca2+ (or Sr2+) concentration was measured in Limulus ventral photoreceptor simultaneously with the receptor potential at 15 °C. The decline of the arsenazo signal has two phases (D1 and D2) when the photoreceptor is bathed in physiological saline.
1. When calcium is replaced by strontium in the superfusate both receptor potential and arsenazo signal are markedly increased in amplitude and the membrane potential is hyper polarized. The decline of the arsenazo signal is prolonged and becomes monophasic; the fast phase D1 of the decline disappears.
2. In strontium saline under voltage clamp conditions the slope of the monophasic decline of the arsenazo signal is the steeper the more negative the membrane voltage.
3. After replacing sodium by lithium in the superfusate the rise of the receptor potential and of the arsenazo signal are not much altered. The decline of the arsenazo signal, however, is slowed down more than 3-fold; this is due to the complete suppression of the fast phase D1 and the retardation of the slow phase D2 of the calcium re-decline.
1. The Na-C a exchanger can accept strontium as a calcium substitute. Strontium has a weaker desensitizing action than calcium. Strontium is not-or only very little -taken up by the endoplasmic cisternae.
2. In sodium-free lithium saline the Na-C a exchanger, the Na-K ATPase and the calcium uptake system of the endoplasmic cisternae do not function. Therefore the intracellular calcium level rises.
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