Abstract
Isolated salivary glands of the cockroach Nauphoeta cinerea Olivier secrete fluid in response to nerve stimulation or application of dopamine, the acinar cells undergoing a hyperpolarization during secretion. The aim of the present work was to examine whether cyclic AMP acts as a second messenger in the acinar cells to cause the secretory and electrical responses to the transmitter dopamine. Cyclic AMP (10-500 microM) in the bathing solution of isolated glands caused a dose-dependent secretory response but no change in the membrane potential of acinar cells. The time courses and magnitudes of the secretory responses to cyclic AMP resembled those features of responses to dopamine. Forskolin, an adenylate cyclase activator, caused fluid secretion but the responses were small and irregular. The phosphodiesterase inhibitor, 3-isobutyl-l-methylxanthine (IBMX)(1-1000 microM) produced fluid secretion in a dose-dependent manner, the maximal response being equal to that of dopamine. Maintained responses to cyclic AMP or IBMX required the presence of extracellular calcium ions. An inhibitor (MDL 12,330A) of adenylate cyclase suppressed the secretory responses to dopamine, cyclic AMP, IBMX, the ionophore A23817 or the readmission of calcium ions to the bathing solution; this inhibitor did not block the acinar hyperpolarization caused by nerve stimulation. Cyclic AMP stimulation of glands, bathed in chloride-free solution to prevent fluid secretion, produced a change in the gland cells which outlasted the period of exogenous cyclic AMP stimulation and expressed itself as a transient secretion upon return of the normal bathing solution. It was concluded that stimulus-secretion coupling in this gland involves a calcium-dependent second messenger system and that cyclic AMP is probably the second messenger. The evidence did not support the idea that cyclic AMP is also a second messenger for the acinar cell hyperpolarization evoked by nerve stimulation.
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