Fig. 3. Cytosolic calcium [Ca²⁺]_i response of rectal gland artery (RGA) to external Ca²⁺ and Gd³⁺ or to spermine. (A) Change in [Ca²⁺]_i of RGA maintained in calcium-free shark Ringer following the addition of Ca²⁺ (approx. 3 mmol l^-1) and then Gd³⁺ (333 µmol l^-1) (values are means ± S.E.M., N=10; ^*P<0.01 for addition of Ca²⁺ compared to baseline [Ca²⁺]_i and ^**P<0.05 for Gd³⁺ compared to Ca²⁺). (B) Representative example of the increase in [Ca²⁺]_i of RGA in calcium-free buffer in response to the addition of spermine (333 µmol l^-1) at the time indicated by the arrow.