The action of inhibitors of protein kinases on fluid and ion secretion by Malpighian tubules of Locusta migratoria,L

Fluid production in Locusta Malpighian tubules was stimulated by corpora cardiaca extract (c. 100%) and dibutyryl cAMP (c. 50%). Chelerythrine and staurosporine (Protein kinase C, PKC inhibitors) inhibited it in the range 0.07-60&mgr;M (IC(50)3&mgr;M), whereas Rp-cAMP (Protein kinase A, PKA inhibitor) caused inhibition over the concentration range 10-1000&mgr;M (IC(50)264&mgr;M). The protein phosphatase inhibitor, okadaic acid, was also inhibitory over the concentration range 0.1-1000nM (IC(50) 91nM). CC extract stimulation increased fluid [Na(+)] from 41 to 59mM and decreased [K(+)] from 127 to 107mM; stimulation with cAMP had no such effect. The PKC inhibitors reduced the [K(+)] in the secreted fluid from 126 to 107mM but had no effect on the [Na(+)]. Subsequent addition of CC extract stimulated fluid production and caused an increase in [Na(+)] from 41 to about 50mM. The addition of Rp-cAMP reduced fluid production but caused a decrease in [Na(+)] from 37 to 28mM and an increase in its [K(+)] from 124 to 148mM. Fluid production by Rp-cAMP inhibited tubules was not stimulated by corpora cardiaca extract or cAMP, but [Na(+)] rose to 36mM. Protein phosphorylation plays a role in the regulation of fluid production probably via the apical and basal membrane cation transporters.