Excretion in the house cricket: role of second messengers in Malpighian tubule function

Abstract

Three factors influencing diuresis, two stimulatory and one inhibitory, have been isolated from the corpora cardiaca of Acheta domesticus by means of high-performance liquid chromatography. Diuretic factor 1 provoked a fast-acting response (15 min to peak), whereas diuretic factor 2 caused a slow-acting (45 min to peak), longer lasting response. When dibutyryl-cAMP was added to the bath it increased secretion in a manner similar to diuretic factor 1. In unstimulated preparations, the divalent cation ionophore A23187 mimicked the action of antidiuretic factor, reducing the secretion rate by approximately 80%. In preparations pretreated with diuretic factor 1 or cAMP, addition of ionophore reduced the secretion rate by 60% or more, but the tubules continued to secrete at the reduced rate for at least 1 h. In contrast, when preparations were pretreated with diuretic factor 2, the addition of A23187 reduced secretion to zero within 15 min. Methanolic extracts of corpora cardiaca increased cAMP levels in tubule broken-cell preparations in a dose-dependent manner over the range of 0.03–1.0 corpus cardiacum equivalents, with a maximal increase of approximately 2 times that of the controls. Similar increases in cAMP levels were produced by homogenates of corpora cardiaca, diuretic factor 1, and sodium fluoride. In contrast, diuretic factor 2 significantly reduced cAMP concentrations. Possible mechanisms by which these three factors interact to regulate tubule secretion are discussed.