The role of calcium in diuretic hormone action on locust Malpighian tubules

Fluid secretion by the Malpighian tubules (MTs) of Locusta is drastically reduced in the absence of extracellular calcium. Verapamil (10^-4 M) inhibits basal secretion, whereas the ionophore A23187 (10^-5 M) elevates the secretory rate. Cyclic guanosine monophosphate (cGMP) stimulates fluid secretion at a concentration of 10^-3 M. A factor extractable in methanol from the storage lobes of the corpora cardiaca stimulates increased guanylate cyclase activity in MTs, resulting in a 10-fold elevation in intracellular cGMP levels. Attempts to separate the factor stimulating guanylate cyclase by high-performance size-exclusion chromatography proved unsuccessful. Neither 5-hydroxytryptamine (5-HT) (10^-4 M) nor A23187 (10^-5 M) are able to elevate intracellular cGMP levels. Elevations of both intracellular cAMP and cGMP levels in response to diuretic hormone (DH) are potentiated in the absence of extracellular calcium. Consistent with these elevations are increases in the rates of fluid secretion by tubules deprived of extracellular calcium. It is concluded that, although calcium has an important role in the regulation of fluid secretion, its role in the mechanism of hormone-stimulated secretion may be modulatory rather than regulatory.