Some effects of insect hormones on Na(^+), K(^+)-ATPase and fluid secretion by the Malpighian tubules of Locusta migratoria L.
A study has been made on the effects of insect hormones on fluid secretion by the Malpighian tubules of Locusta, and on the Na(^+), K(^+)- ATPase activity in microsomal preparations of the tubules. A diuretic hormone present in extracts of the neurosecretory cells and corpora cardiaca accelerated rates of fluid secretion by in vitro preparations of the tubules but had no effect on ATPase activity. Ecdysone affected neither secretory rates nor enzyme activity whereas Juvenile Hormone had an inhibitory effect on both. Attempts have been made to explain how. J.H. may inhibit Na(^+), K(^+) -ATPase activity. It is possible that J.H. affects the membrane conformation and thus prevents the normal reaction sequence of the ATPase. Ultrastructural studies have shown that the fine structure of the Malpighian tubules varies with development. Invaginations of the basal and apical cell membranes were found to develop with increasing age throughout the 5th stadium. At the same time the numbers of mitochondria in the tubule cells appeared to increase and the mitochondria came to lie in the cytoplasm of the basal infolds. Just prior to the larval-adult moult the invaginations of both membranes decreased and mitochondria were rarely found amongst the basal infolds. Associated with these ultrastructural changes, functional changes are also reported. Na(^+) , K(^+)-ATPase activity was low at the beginning and. end of the 5th stadium, times when there was least invagination of the plasma membrane. At the same times animal relative water content was high, suggesting lower rates of secretion. Both ouabain and ethacrynic acid were found to inhibit fluid secretion by, an. d Na(^+), K(^+)-ATPase, in microsomal preparations of the Malpighian tubules. Ouabain was the more effective inhibitor of enzyme activity ( pI(_50) = 5.8 as compared with ethacrynic acid pI(_50) = 2.5).