Homogenates of whole corpora cardiaca (CC) cause increases in the short-circuit current (Isc) and transepithelial electropotential difference (PD) across locust recta of 3-fold and 1·7-fold respectively, in comparison with the values for unstimulated steady-state recta. Maximum stimulation restores rectal Isc and PD to levels observed immediately after removing this organ from animals. Cyclic-AMP causes a similar maximum increase in Isc and PD; however, the response exhibits a much shorter lag time and a faster rate of rise than is observed for stimulation with CC. The addition of CC to the haemocoel side of everted rectal sacs caused whole tissue levels of cAMP in this organ to increase 3-fold. The relationship between the logarithm of CC or cAMP concentration and the increase in Iscis linear, and the decline in ΔIsc with time is also dose dependent. Small maximum increases in Isc are caused by homogenates of ventral ganglia, whole brain and rectal tissue, but the concentration of the stimulatory activity in these locust tissues is clearly three orders of magnitude lower than in CC. Inhibitors of HCO3−/H+ and Cl− transport in vertebrate systems, acetazolamide and thiocyanate, do not inhibit the stimulation of recta by CC or cAMP.
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Spring et al. (1980) studied this question.
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