Control of ion and fluid transport by putative second messengers in different segments of the Malpighian tubules of the black field cricket Teleogryllus oceanicus

The differences in second messenger control of secretion were investigated in the distal and main segments of the Malpighian tubules of the black fiel d cricket Teleogryllus oceanicus. Secretion by the main segment was conside rably increased by corpora cardiaca extract and db-cAMP. Corpora cardiaca h ad no effect on secreted fluid composition or intracellular elemental compo sition but db-cAMP increased Na+ and Cl- transport, as measured by x-ray mi croanalysis of secreted fluids and cells. Secretion by the main segment was considerably increased by forskolin and by Sp-cAMP. Secretion in the dista l segment was abolished by corpora cardiaca extract but was unaffected by d b-cAMP and only slightly reduced by 8-bromo-cAMP. However, Sp-cAMP increase d secretion but forskolin reduced secretion. The responses of the distal se gment suggest the possibility of a multiplicity of controls through differe nt protein kinases and adenylyl cyclases. Secretion rate in the main segmen t was also increased by cGMP but distal segment secretion was unaffected. S ecretion from both segments was increased by 5-HT. In the main segment secr etion rate was increased by Ca-ionophore and thapsigargin and decreased by verapamil. This suggests a role for Ca2+ as a controlling second messenger. In the distal segment only Ca-ionophore had an effect on secretion rate, w hich was reduced. Secretion rates in both segments were decreased in Ca-fre e saline. In saline in which Sr2+ replaced Ca2+, secretion rate in the main segment was greatly increased whilst that of the distal segment was decrea sed, suggesting that Sr2+ could substitute for Ca2+ as a second messenger. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.