Dopamine-induced graded intracellular Ca2+ elevation via the Na+Ca2+ exchanger operating in the Ca2+-entry mode in cockroach salivary ducts

Stimulation with the neurotransmitter dopamine causes an amplitude-modulated increase in the intracellular Ca2+ concentration ([Ca2+]i) in epithelial cells of the ducts of cockroach salivary glands. This is completely attributable to a Ca2+ influx from the extracellular space. Additionally, dopamine induces a massive [Na+]i elevation via the Na+K+2Cl− cotransporter (NKCC). We have reasoned that Ca2+-entry is mediated by the Na+Ca2+ exchanger (NCE) operating in the Ca2+-entry mode. To test this hypothesis, [Ca2+]i and [Na+]i were measured by using the fluorescent dyes Fura-2, Fluo-3, and SBFI. Inhibition of Na+-entry from the extracellular space by removal of extracellular Na+ or inhibition of the NKCC by 10 μM bumetanide did not influence resting [Ca2+]i but completely abolished the dopamine-induced [Ca2+]i elevation. Simultaneous recordings of [Ca2+]i and [Na+]i revealed that the dopamine-induced [Na+]i elevation preceded the [Ca2+]i elevation. During dopamine stimulation, the generation of an outward Na+ concentration gradient by removal of extracellular Na+ boosted the [Ca2+]i elevation. Furthermore, prolonging the dopamine-induced [Na+]i rise by blocking the Na+/K+-ATPase reduced the recovery from [Ca2+]i elevation. These results indicate that dopamine induces a massive NKCC-mediated elevation in [Na+]i, which reverses the NCE activity into the reverse mode causing a graded [Ca2+]i elevation in the duct cells.