Studies on ion movements in Malpighian tubules of Locusta migratoria L., with particular reference to their endocrine control
Intracellular microelectrodes were used in conjunction with ion substitution, agonists and inhibitors of known transport processes and diuretic hormone (DH), to investigate the mechanisms whereby ions cross the basal and apical cell membranes of the Malpighian tubules of Locusta migratoria, and their endocrine control. Values for basal, apical and transepithelial potentials in control saline were -70.4 + 0.9 mV, -76.1 ± 1.5 mV and +5.5 ± 1.3 mV (lumen positive), respectively (n=73).Ion substitution experiments, involving K(^+), Na(^+) and CI(^-) in the bathing medium, indicated that the basal membrane is more permeable to K(^+) than Na(^+) and CI(^-). Ion flux studies suggest that CI(^-) entry across the basal membrane is not by direct cotransport with K(^+), nor Na(^+). Some CI(^-) entry also appears to be stimulated under conditions of high K(^+)(_o). Crude corpus cardiacum (CC) preparations with DH activity effected ca. 150% increase in tubular fluid secretion above basal secretion in vitro, and ca. 106% and 335% increase in net transepithelial Na(^+) and Cl(^-) movement from bathing medium to lumen above basal flux, respectively. Treatment of tubules with CC extract also resulted in a significant increase in levels of the intracellular second messengers, cAMP and the Ca(^2+)-mobilizing Inositol 1,4,5- trisphosphate. Ion substitution experiments using dibutyryl cAMP suggest that cAMP stimulates an apical cation pump, whilst having no effect on Cl(^-) conductance. Cytochemical localization and membrane separation techniques used in conjunction with biochemical analyses indicate the presence of (Na(^+)+K(^+))-ATPase on the basolateral membranes, and HCO(_3)- stimulated ATPase on both the basolateral and apical membranes. The results referred to above are discussed, and a hypothetical model is proposed to describe the endocrine control of ion movements across the two cell membranes mediated by changes in intracellular cAMP and Ca(^2+).