Medullopressin: a new pressor activity from the renal medulla

Altered renal medullary blood flow: A key factor or a parallel event in control of sodium excretion and blood pressure?

In the context of the ongoing debate on the mechanism of blood pressure (BP) regulation and pathophysiology of arterial hypertension ("renocentric" vs "neural" concepts), attention is focused on the putative regulatory role of changes in renal medullary blood flow (MBF). Experimental evidence is analysed with regard to the question whether an elevation of BP and renal perfusion pressure (RPP) is likely to increase MBF due to its impaired autoregulation. It is concluded that such increases have been clearly documented only in rats with extracellular fluid volume expansion. A possible translation of this finding to BP regulation in health and hypertension in humans may only be a matter of speculation. Within the "renocentric" theory, the key event leading to restoration of initial BP level is pressure natriuresis. Its relation to elevation of renal interstitial hydrostatic pressure and to the phenomenon of "wash-out" of renal medullary solutes by increasing MBF is discussed. We also assessed the validity of data supporting the putative mechanism of short-term restoration of elevated BP owing to the release of a vasodilator lipid (medullipin) by the medulla. The structure of the proposed medullary lipid is still undefined, and there is no sound evidence on its mediatory role in lowering elevated BP level. In conclusion, MBF change can hardly be regarded as a crucial event in the regulation of BP: it can be involved in the control of sodium excretion and BP only in some circumstances, although its contributory role cannot be excluded.

The vasodepressor function of the kidney: further characterization of medullipin and a second hormone designated angiolysin

The objective of this study was to further characterize the antihypertensive properties of medullipin and a second hormone designated angiolysin physiologically. Angiolysin and medullipin were tested in coronary and aortic rings from cows, sheep, pigs, mice and rats. In vivo animal experiments were performed using spontaneously hypertensive rats. Medullipin was successfully separated from another antihypertensive agent at the polar end of the polarity continuum. It is an extremely potent vasodilator. With the methods available today, it is not possible to make a galenical preparation of medullipin for in vivo analysis. The newly discovered antihypertensive agent is another extremely potent vasodilator, even stronger than medullipin, and was therefore named angiolysin. The vasodilatory activity of both medullipin and angiolysin persisted for hours, on rat and mouse aortae, and on the coronary arteries of pigs, cows and sheep. Both substances exerted their effects even in animal rings that were precontracted with 100 mmol/l K+. Angiolysin reduced the resting tension in blood vessels from mice and rats even without precontraction. A single injection of angiolysin resulted in a dose-dependent reduction of blood pressure, independent of the initial blood pressure, even to zero if the dosage was sufficient. The effect persisted for several hours. In conclusion, both hormones are extremely potent vasodilators, and are expected to lead to paradigmatic changes in the treatment of hypertension. With regard to potency, only sodium nitroprusside is comparable, but the effects of medullipin and angiolysin persist for hours after a single injection.

The vasodepressor function of the kidney: prostaglandin E2 is not the principal vasodepressor lipid of the renal medulla

AIM

Whereas prostaglandin E2 has been characterized as the principal vasodepressor lipid, medullipin remains a hypothetical vasodepressor principle of the renal medulla. Representing the first step towards the isolation of medullipin as a pure compound, the aim of the present study was to determine whether or not the known vasodilator and antihypertensive action of prostaglandins play a role in the antihypertensive activity of renal medulla.

METHODS

A chloroform extract of porcine kidney medulla was fractionated by gradient vacuum liquid chromatography (VLC) and analysed by capillary GC-MS for the presence of prostaglandins (detection limit: 2.2 ppm). The biological activity was determined in spontaneously hypertensive Wistar rats. The particle size of injectable colloids prepared from extract and fractions was controlled by photon correlation spectroscopy.

RESULTS

The extract caused a pronounced blood pressure decline (29.6 +/- 6.3/24.9+/- 5.5 mmHg; P = 0.0078; 10 mg kg(-1) body weight; particle size of 143 +/- 18 nm; n = 7) lasting for more than 1 h. The heart rate remained stable, showing only a slightly decrease. All fractions were shown to be devoid of vasodilator prostanoid substances. The VLC procedure allowed the successful separation of endogenous emulsifiers from the active principle. An extract from the renal cortex did not exhibit a similar vasodepressor effect.

CONCLUSION

Prostaglandins are excluded as the blood pressure-lowering active principle of a total lipid kidney medulla extract. The vasodepressor principle is contained in the kidney medulla, but not in the cortex. It can be separated from endogenous emulsifying substances, is chromatographically stable, and is amenable to purification and chemical characterization.