Adrenal medullary adaptations and cardiovascular regulation after 6-hydroxydopamine treatment in rats

Experiments were conducted to assess the pattern of activation of adrenal medullary adrenaline (ADR) and noradrenaline (NA) chromaffin cells following damage to sympathetic neurons with 6-hydroxydopamine (6-HDA, 100 mg/kg per day for 7 days, s.c.). 6-HDA treatment resulted in a sustained elevation in urinary ADR excretion, suggesting that the adrenal medulla was continuously stimulated. Further evidence of neural stimulation of the adrenal medulla was demonstrated by a persistent elevation of adrenal medullary dopamine (DA) content, an indicator of catecholamine synthesis. The increase in synthetic rate was associated with a significantly augmented medullary NA and ADR content. However, NA content was elevated after only 1 day of treatment, whereas, ADR became elevated after the second day, a finding that suggests a differential activation of the two populations of chromaffin cells. Cardiac and kidney NA contents were reduced by 97 and 87%, respectively, and BP increments to stimulation of the thoracolumbar spinal cord of pithed rats (0.5 to 16 Hz) were attenuated by more than 75% by 6-HDA. Moreover, the residual BP responses were abolished by adrenalectomy. Thus the results indicate that the changes in adrenal CA disposition may be related to a greater reliance upon the adrenal medulla to maintain blood pressure (BP) following damage to sympathetic neurons, and further, the findings are consistent with the possibility that ADR and NA chromaffin cells are separately controlled.