Figure 4. Changes in somatic [Ca²⁺]_i in response to repeated applications of CtVm.

A, dual CtVm exposure in nASW. Application of 100 μg ml⁻¹ CtVm produced a typical elevation of bag cell neurone somatic [Ca²⁺]_i in nASW. Following recovery to steady state and wash in nASW, CtVm was applied again, but had no effect on [Ca²⁺]_i, i.e. the neurone was refractory. B, average data for four such experiments showing that while the initial CtVm exposure (CtVm 1) produced a significant elevation of ∼150 nM in somatic [Ca²⁺]_i, a subsequent exposure (CtVm 2) had essentially no effect on Ca²⁺ levels. Since the peak-type response was not elicited by the first CtVm exposure in all neurones tested in this manner, only changes in steady-state [Ca²⁺]_i are presented. C, recovery from CtVm-induced refractoriness over 24 h. A bag cell neurone exhibited a standard somatic [Ca²⁺]_i elevation in response to 100 μg ml⁻¹ CtVm in nASW (^). The CtVm was then washed out and the neurone returned to the incubator for ∼24 h. When the CtVm was applied again on day 2, the [Ca²⁺]_i rose to similar steady-state levels seen on day 1 (∼400 nM; •). D, the collective data for 13 experiments shows that CtVm is capable of elevating somatic [Ca²⁺]_i in nASW on both days 1 and 2. If a peak-type response is observed on day 1, it is invariably absent on day 2; consequently, only the steady-state changes in [Ca²⁺]_i are quantified. The steady-state levels reached during day 2 CtVm exposure are typically less than that achieved on day 1.