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This study evaluated the transduction pathway associated with type 3 Na⁺/H⁺ exchanger (NHE3) activity-induced inhibition during dopamine D₃ receptor activation in immortalized renal proximal tubular epithelial cells from the spontaneously hypertensive rat. The dopamine D₃ receptor agonist 7-OH-DPAT decreased NHE3 activity, which was prevented by the D₂-like receptor antagonist S-sulpiride, pertussis toxin (PTX; overnight treatment), and the PKC inhibitor chelerythrine, but not by cholera toxin (overnight treatment), the MAPK inhibitor PD-098059, or the p38 inhibitor SB-203580. The PKA inhibitor H-89 abolished the inhibitory effects of forskolin on NHE3 activity, but not that of 7-OH-DPAT. The phospholipase C (PLC) inhibitor U-73122 prevented the inhibitory effects of 7-OH-DPAT, whereas PDBu and 7-OH-DPAT increased PLC activity and reduced NHE3 activity; downregulation of PKC abolished the …