Fig. 7. Changes in [Ca²⁺]_i of rectal gland tubules (RGTs) in response to calcium, spermine and gadolinium treatment. (A) The stimulatory effect of spermine (N=7) on [Ca²⁺]_i in RGT was greater than that achieved by Gd³⁺ (N=10) (^*P<0.01 compared to baseline, ^**P<0.01 for spermine versus calcium, P<0.05 for Gd³⁺ versus Ca²⁺). Values are means ± S.E.M. (B) Representative example of the response of RGT in calcium-free shark Ringer to [Ca²⁺]_e followed by spermine. (C) Representative example of RGT showing the stimulatory effect of sequential addition of external Ca²⁺ and Gd³⁺ at the times indicated by the arrows.