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Ion transport and short circuit current in the rectum of the desert locust, Schistocerca gregaria Williams, Douglas Lloyd

Abstract

An in vitro preparation of the rectum of the desert locust Schistocerca gregaria is described and characterized. The rectal epithelium was mounted as a flat sheet separating two well stirred chambers and the trans-membrane potential difference (PD), membrane resistance, and short circuit current (SCC) were monitored. The PD and membrane resistance remained relatively constant for at least six hours at 35 mV (lumen positive) and 6000 Ohms . cm ⁻², respectively. After an initial two hour transient period during which the SCC dropped from 8 uMoles of charge .cm⁻² .hr⁻¹ to 3.5 uMoles. cm⁻² .h⁻¹ an approximate steady-state condition was reached and maintained for at least another four hours. The SCC was consistant with either a net cation transport from the hemolymph to the lumen of the rectum or a net anion transport in the opposite direction. Since Cl⁻ was the only major anion in the bathing media some experiments were carried out to evaluate the contribution of active Cl⁻ transport to the SCC. Substitution of SO4⁻ of NO3⁻ during the third and fourth hours in vitro had no effect on the SCC, although they abolished the initial two hours transient. Similar experiments in which Cl⁻ was substituted by acetate had a complex stimulatory effect on the SCC. Measurements of ³⁶Cl⁻ fluxes under SCC conditions during the same time period demonstrated a mean net flux of 1.52 uMoles of Cl⁻ cm⁻².hr from the rectal lumen to the hemolymph. This value is equivalent to one half the simultaneous SCC. These experiments clearly demonstrate that the rectal epithelium actively transports Cl⁻ in a direction consistent with the observed SCC but that this active transport of Cl⁻ does not contribute to the SCC during the third and fourth hours in vitro. Since rectal Na+ and K+ transport reported by other workers is in the wrong direction to account for the observed SCC, it seems necessary to propose (1) a Cl⁻ exchange pump (probably with HCO3⁻) perhaps located on the luminal membrane to account for the fact that active Cl⁻ transport does not contribute to the SCC and (2) a H+ and/or HCO3⁻ pump to account for the SCC. These proposals are incorporated in a model for organization of ion transport processes in the locust rectum.

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