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 Ca(2+) concentration (Ca(2+)) in epithelial cells of the ducts of cockroach salivary glands. This is completely attributable to a Ca(2+) influx from the extracellular space. Additionally, dopamine induces a massive Na(+) elevation via the Na(+)K(+)2Cl(-) cotransporter (NKCC). We have reasoned that Ca(2+)-entry is mediated by the Na(+)Ca(2+) exchanger (NCE) operating in the Ca(2+)-entry mode. To test this hypothesis, Ca(2+) and Na(+) 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 microM bumetanide did not influence resting Ca(2+) but completely abolished the dopamine-induced Ca(2+) elevation. Simultaneous recordings of Ca(2+) and Na(+) revealed that the dopamine-induced Na(+) elevation preceded the Ca(2+) elevation. During dopamine stimulation, the generation of an outward Na(+) concentration gradient by removal of extracellular Na(+) boosted the Ca(2+) elevation. Furthermore, prolonging the dopamine-induced Na(+) rise by blocking the Na(+)/K(+)-ATPase reduced the recovery from Ca(2+) elevation. These results indicate that dopamine induces a massive NKCC-mediated elevation in Na(+), which reverses the NCE activity into the reverse mode causing a graded Ca(2+) elevation in the duct cells.