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Studies on the hormonal regulation of ion resorption in Schistocerca gregaria Spring, Jeffrey Herbert

Abstract

When isolated locust recta are short-circuited in 'Ussing-type' chambers, they exhibit an initial decline in short-circuit current (Isc) and transepithelial electropotential difference (PD) over the first two hours. Following this decline, recta remain in a steady-state condition for at least 4 h, during which these parameters decline only slightly. There is a factor present in the corpora cardiaca (CC) of schistocerca gregaria which causes the Isc and PD of short-circuited recta to increase. Maximum stimulation restores rectal Isc and PD to the levels observed immediately after removing this organ from locusts. Cyclic AMP causes a similar maximum increase in Isc and PD although the response exhibits a much shorter lag-time and a faster rate.of increase than with CC. The relationship between the logarithm of CC dose or cAMP concentration and the maximum ΔIsc is linear. Maximum stimulation is achieved with 0.05 pr CC or 0.3 mM cAMP, and the decline in Δlsc is dose-dependent and occurs over a matter of hours. Inhibitors of HCO₋₃/H+ and Cl- transport in vertebrates do not inhibit the stimulation of recta by CC or cAMP. Putative neurotransmitter substances and homogenates of flight muscle do not change rectal Isc or PD. Although homogenates of whole brains, ventral ganglia and rectal tissue cause small increases in Isc, the concentration of active factor is nearly three orders of magnitude greater in the CC. The unidirectional and net fluxes of ²²Na+ across short-circuited recta remain constant with time and are unaffected by the initial decline in Isc or by stimulation with CC or cAMP. Net ³⁶Cl- flux closely matches the Isc over the entire experiment, following the initial decline in Isc and rising when recta are stimulated with CC or cAMP. The stimulation of active net Cl- uptake is sufficient to account for the entire increase in Isc. Addition of CC to everted rectal sacs causes the levels of cAMP in this tissue to triple within 15 minutes. Removal of Cl- from the bathing media inhibits any stimulation by CC or cAMP, but the response returns as soon as Cl- is restored. Experiments using complex Cl-free salines suggest that alternate electrogenic transport processes can be turned on, and supported, by complex media. When starved locusts are fed lettuce, their haemolymph stimulates rectal Isc and PD in a similar manner to a submaximal dose of CC and the increase in Isc can be completely accounted for by the increase.in net ³⁶Cl- flux, as is the case with CC and cAMP. Removal of the CC from live locusts reduces or eliminates this feeding response. Further experiments suggest that the CC factor, which I have called Chloride-Transport Stimulating Hormone (CTSH), is not one of the known CC hormones. Although CC and haemolymph samples can increase water resorption in rectal sacs, the action of CTSH does not seem to be directly linked to antidiuresis. Structurally, it is a small protein (M.W. 10,000), water-soluble and relatively heat-stable. To date, I cannot detect any difference between the CC and haemolymph factors, although further experiments to purify CTSH from both sources are in progress.

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