EFFECTS OF RILMENIDINE ON PROXIMAL TUBULAR FLUID ABSORPTION IN RATS

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Contribution of imidazoline receptors and alpha2-adrenoceptors in the rostral ventrolateral medulla to sympathetic baroreflex inhibition by systemic rilmenidine

OBJECTIVES

To determine whether the hypotensive and sympathetic baroreflex inhibition by rilmenidine administered systemically are mediated via imidazoline receptors in the rostral ventrolateral medulla (RVLM).

METHODS

Initial dose-response curves to rilmenidine were determined in urethane anaesthetized rabbits. Effects of a single intravenous dose of rilmenidine (445 microg/kg) on the renal sympathetic nerve activity (RSNA) baroreflex were examined before and after microinjection into the RVLM of the mixed imidazoline/alpha2-adrenoceptor antagonist idazoxan and the alpha2-adrenoceptor antagonist 2-methoxyidazoxan (2-MI).

RESULTS

Intravenous administration of rilmenidine lowered mean arterial pressure and RSNA, inhibited the RSNA baroreflex range by 33% and shifted the baroreflex curve to the left. Idazoxan injected into the RVLM reversed the hypotension and completely restored the baroreflex curve at doses that did not affect the hypotension produced by the selective alpha2-adrenoceptor agonist alpha-methylnoradrenaline. The alpha2-adrenoceptor antagonist, 2-MI also reversed the rilmenidine sympatho-inhibition suggesting that alpha2-adrenoceptors are activated as well.

CONCLUSIONS

The results of the present study show that the hypotensive and sympatho-inhibitory actions of systemic rilmenidine are primarily mediated via imidazoline receptors in the RVLM. However, alpha2-adrenoceptors are also involved, probably as a direct result of the imidazoline receptor action.