THE EFFECTS OF EXTERNALLY APPLIED LITHIUM AND STRONTIUM ON THE ARSENATE-MONITORED CYTOSOLIC CALCIUM SIGNAL OF THE LIMULUS VENTRAL PHOTORECEPTOR

The intracellular arsenate signal indicating the transient light-evoke d change in cytosolic Ca2+ (or Sr2+) concentration was measured in Lim ulus ventral photoreceptor simultaneously with the receptor potential at 15 degrees C. The decline of the arsenate 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 arsenate signal are markedly increased in amplitude and the membrane potential is hyperpolarized. The decline of the arsenate signal is prolonged and becomes monophasic; the fast phase D1 of the decline disappears. 2. In strontium saline under voltage clamp conditi ons the slope of the monophasic decline of the arsenate signal is the steeper the more negative the membrane voltage. 3. After replacing sod ium by lithium in the superfusate the rise of the receptor potential a nd of the arsenate signal are not much altered. The decline of the ars enate 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.