Inhibitors of Na+/Mg2+ exchange activity attenuate the development of hypertension in angiotensin II-induced hypertensive rats

The paper by Touyz and Yao [1] in this issue of the journal concerns two topics that have been part of hypertension research for several decades. These are the renin–angiotensin system and intracellular cation concentrations. There have been many ideas on these topics and Touyz and Yao put forward another suggestion as to how angiotensin II could alter cell ion concentration, in this case increased sodium and decreased magnesium, and argue that this could mediate the rise in blood pressure. These conclusions are drawn from the observations that (i) blood pressure elevation caused by angiotensin II is accompanied by increased platelet sodium and decreased platelet magnesium and (ii) that treatment with either of two Na+/Mg+ exchange inhibitors modifies both blood pressure and cell ion responses to angiotensin II. Whether these coincident effects indicate causation deserves closer scrutiny. The observation that angiotensin II causes an increase in cell sodium in this study is consistent with previous reports [2]. However, the use of inhibitors to try to elucidate the pathway that mediates the increased sodium uptake into cells has yielded inconsistent results. There is evidence that angiotensin II stimulates Na+/H+ exchange, causing an increase in sodium influx to cells [2]. In such a study, an inhibitor believed to be specific to the Na+/H+ exchanger prevented a rise in intracellular sodium in epithelial cells which supported this transporter as the mediator of the angiotensin II-induced increase in cell sodium [2]. However, angiotensin II was shown to increase NaK2Cl cotransport and frusemide was reported to inhibit the angiotensin II-stimulated increase in intracellular sodium in macula densa cells [3]. The work by Touyz and Yao [1] suggests that inhibition of Na+/Mg2+ countertransport in platelets also prevents the increase in cell sodium caused by angiotensin II. Clearly, if blockade of one transporter stops the increase in cell sodium, that transporter must be solely responsible for the effect and this cannot be true for all three. Alternatively, angiotensin II action is mediated by a different transporter in each cell type although the AT1 receptor appears to be responsible in each case. It should also be pointed out that increases in sodium influx by such facilitative pathways are normally matched by increases in Na+/K+ ATPase activity to maintain a steady intracellular sodium concentration. Previous work from Touyz's laboratory has shown the effect of angiotensin II on Na+/H+ exchange and indicated that the latter transporter has an effect on Na+/Mg2+ countertransport [4]. Increasing cell sodium by increased Na+/H+ exchange activity should tend to decrease Na+/Mg2+ countertransport as a result of a lower Na+ gradient driving force. Therefore, a secondary effect of Na+/H+ exchange inhibition to decrease Na+/Mg2+ exchange activity seems unlikely by this mechanism.This argues for a direct effect of angiotensin II to increase Na+/Mg2+ exchange activity by some means. However, in this case, inhibition of neither Na+/H+ nor Na+/Mg2+ alone should abolish the increase in cell sodium, as seen with dimethylamiloride previously [2] or with quinidine in the current paper by Touyz and Yao [1]. Thus, the mechanism by which angiotensin II causes an increase in cell sodium remains open to doubt. However, it appears likely that angiotensin II causes a decrease in cell free magnesium by activation of Na+/Mg2+ exchange. The role of cell magnesium requires careful consideration. Most magnesium in the cell is bound to sites such as ATP and proteins and this is the functional component. The extent to which it is measured depends on the relative binding affinity of these sites in competition with the fluorescent probe. A change in bound magnesium with no change in free magnesium has been reported in essential hypertensive patients (EHT) [5], presumably due to a change in the binding sites. In platelets from spontaneously hypertensive rats (SHR) [6] and from EHT [7], free cell magnesium has been found to be increased whereas in vascular smooth muscle (VSM) from SHR, it has been found to be decreased [8], although this was total rather than free magnesium. However, magnesium has also been reported to be increased in VSM from SHR, but only after 8–10 months, whereas cell sodium was increased several months earlier [9], indicating that these ions had changed by independent mechanisms. Clearly, blood pressure can be increased in conditions leading to an increase in platelet magnesium. If blood pressure can be equally raised by conditions that cause a decrease in platelet magnesium, as shown by Touyz and Yao [1], is it reasonable to propose that the change in cell magnesium is not of central importance? The above observation is pertinent to the second question examined by Touyz and Yao, which is whether the cell ion changes cause the angiotensin II-stimulated rise in blood pressure. Because the increase in sodium and decrease in magnesium occur together in their experiments, the possible individual effects of these ions on blood pressure cannot be distinguished. Imipramine only prevents approximately one-half the change in cell ions induced by angiotensin II whereas quinidine gives total blockade of the angiotensin II cell ion changes. Despite this, imipramine is at least and probably more effective than quinidine at blocking the angiotensin II-induced rise in blood pressure. This lack of quantitative association lessens confidence in the proposition of causality. The alternative proposition is that angiotensin II alters cell ions and blood pressure by independent means and that imipramine and quinidine similarly antagonize the angiotensin II-induced rise in blood pressure independently from their effect on the change in cell ions. It is interesting that, almost 20 years ago, it was reported that quinidine had no effect on the response to angiotensin II but it was a potent antagonist of α-adrenergic stimulation [10]. Quinidine has also been shown to inhibit ATP stimulated contractions via P2x purinoceptors [11], to block potassium channels [12] and to cause baroreflex mediated increases in sympathetic nerve activity [13]. Activation of phospholipase D is one of the several signalling pathways of angiotensin II [14,15] and imipramine has been shown to affect phospholipase D and antagonizes its activation by other agents [16]. Plasma sodium concentration is a function of water balance and the increase in plasma sodium concentration in the experiments shown by Touyz and Yao [1] suggests a change in water balance in the rats. However, this did not occur until 1 week after the maximal rise in blood pressure and its cause is unclear. Angiotensin II has a direct effect on thirst but it normally increases thirst and hence water intake [17], which might be expected to decrease plasma sodium concentration. It seems safe to believe that angiotensin II affects Na+/Mg2+ exchange to alter cell magnesium but whether the change in cell sodium is by the same transporter is uncertain. In addition, the suggestion that the cell ion changes cause a rise in blood pressure is only one of several tenable hypotheses and it may not be the most likely one.