Organ specificity of the dopamine₁ receptor/adenylyl cyclase coupling defect in spontaneously hypertensive rats

The coupling between the dopamine₁ (DA₁) receptor and the G protein/adenylyl cyclase (AC) enzyme complex is defective in the proximal convoluted tubule (PCT) of 20-wk-old spontaneously hypertensive rats (SHRs). Because this coupling defect could have been due to desensitization secondary to elevated renal dopamine levels in the adult animal, we studied the interaction between DA₁ receptors and AC in PCT of rats as early as 3 wk of age, a time when renal dopamine levels are similar in SHRs and their normotensive controls (Wistar-Kyoto rats, WKYs). Maximum receptor density did not change with age and was similar in WKYs and SHRs in all the age groups studied (3, 8, and 20 wk). Basal-, forskolin-, and guanyl nucleotide- stimulated AC activities were also similar in WKYs and SHRs and did not change with age. However, the DA₁ agonist-stimulated AC activity was greater in WKYs than in SHRs and increased with age in WKYs but not in SHRs. Moreover, the ability of a nonhydrolyzable analogue of GTP, Gpp(NH)p, to enhance DA₁ agonist (SND-919-C12, 1 μM)-stimulated AC activity increased with age in WKY but not in SHRs. To determine if the defect noted in the PCT of SHRs is due to a defective D(1A) receptor gene, parallel studies were performed in the striatum, since this receptor is expressed predominantly in the latter tissue. In contrast to the results in PCT, radioligand binding and AC studies in striatum revealed no differences between WKYs and SHRs. The D(1A) receptor gene was expressed in the striatum and proximal tubules of both WKYs and SHRs; there were no differences as determined by ribonuclease protection assay. We conclude that there is a primary defect in the DA₁ receptor-G protein coupling mechanism in the PCT of SHRs. This defect does not seem to reside in the D(1A) receptor gene.