Dietary Ca2+ and blood pressure: evidence that Ca2+‐sensing receptor activated, sensory nerve dilator activity couples changes in interstitial Ca2+ with vascular tone

It has been more than 20 years since Ayachi [1] showed that feeding Ca2+ to the spontaneously hypertensive rat lowers the animal's blood pressure. This finding, along with other seminal observations [2–4], sparked a burst of activity exploring the relationship between dietary Ca2+ intake and blood pressure regulation in human and animal models of hypertension. While the question of whether elevated Ca2+ intake lowers blood pressure in humans remains controversial [5,6], data showing an inverse correlation between Ca2+ intake and blood pressure in several models of hypertension in the rat and dog are consistent [7–11]. Over the past two decades, multiple mechanisms have been postulated to explain how increased Ca2+ intake lowers blood pressure. These range from diet‐induced suppression of serum levels of calciotropic hormones, including 1,25 (OH)2 vitamin D3 and parathyroid hormone [12], to suppression of sympathetic vasoconstrictor tone through modulation of central [7] or peripheral nervous system activity [13]. One hypothesis that emerged held that elevated Ca2+ intake reduces blood pressure by increasing serum ionized Ca2+ levels, which in turn cause peripheral vasodilation [14]. This idea was based on two observations. One was the finding that feeding rats a high Ca2+ diet caused a slight, but significant, increase in serum ionized Ca2+ [2,11]. The second was the work of David Bohr and colleagues showing that raising the concentration of Ca2+ in the solution bathing isolated arteries caused vasorelaxation [15,16]. The overall hypothesis was generally regarded as untenable, however, because of the discrepancy between the very small changes in serum ionized Ca2+ that were observed in animals (0.2 mmol/l) and the large (>2 mmol/l) increases in extracellular Ca2+ that were required to induce relaxation of isolated arteries. Over the past few years we have performed a series of studies that demonstrate that isolated mesenteric branch arteries of the rat and mouse undergo nearly complete relaxation when the concentration of extracellular Ca2+ in the bathing media is raised from 1 to 2 mmol/l [17,18]. This work represents an important extension beyond previous reports of Ca2+‐induced relaxation because the relaxation is seen at much lower concentrations of Ca2+ than had previously been observed. Critical to our demonstration of high sensitivity Ca2+‐induced relaxation was our discovery that the relaxation event is dependent on an intact perivascular sensory nerve network. For example, we have found that extensive dissection of perivascular connective tissue, acute or sub‐chronic denervation with ethanolic phenol [17], or chronic sensory denervation [19] all significantly attenuate the relaxation induced by elevation of extracellular Ca2+. Moreover, we have found that Ca2+‐induced relaxation is antagonized by pharmacological manoeuvres that inhibit hyperpolarization‐mediated relaxation and specifically by blockers of high conductance potassium–calcium channels [20]. These observations have led to the hypothesis that exposure of the sensory nerve ending to extracellular Ca2+ induces the release of a hyperpolarizing vasodilator substance [17]. The finding that extracellular Ca2+ causes significant relaxation between 1 and 2 mmol/l, combined with the demonstration of a periadventitial nerve dependence of the relaxation event, led to our hypothesis that the perivascular sensory nerve network monitors changes in ionized Ca2+ in the interstitial fluid compartment of tissues that are involved in transcellular Ca2+ movement, i.e. the small bowel, kidney and bone [21]. Thus, an increase in Ca2+ in the interstitial space of one of these tissues would be associated with vasodilation, increased regional blood flow and a washout of Ca2+ from the tissue compartment. To test this hypothesis, we used an in situ microdialysis method to measure interstitial Ca2+ in the intestinal submucosa and the renal cortex under basal conditions and during manipulation of Ca2+ homeostasis. The results of the studies performed with the small intestine showed that the concentration of interstitial Ca2+ in the duodenal submucosa changes dynamically as a function of the concentration of Ca2+ present in the lumen of the gut, rising from 1.08 to 1.9 mmol/l when Ca2+ in the small bowel is raised from 0 to 6 mmol/l [18]. We also found that the level of Ca2+ in the duodenal submucosa was sensitive to the feeding status of the animal, ranging from 1.1 mmol/l in the 24‐h‐fasted rat to 1.45 mmol/l in the free‐feeding rat consuming a 1% Ca2+ diet. When the kidney was studied we found that the concentration of free Ca2+ in the renal cortex was ∼1.65 mmol/l under basal conditions and increased 35–40% in response to either raising the concentration of Ca2+ in the small bowel or after a 1 h infusion of parathyroid hormone [22]. Thus, Ca2+ in the interstitial compartment of at least two tissues that contribute significantly to peripheral resistance, and blood pressure regulation varies dynamically over a range that elicits relaxation of isolated arteries. The data discussed above indicates that high sensitivity Ca2+‐induced relaxation occurs over a physiological concentration range and is sensory nerve dependent. These observations suggest that the sensory nerve network has a mechanism for monitoring the concentration of Ca2+ that is present in the surrounding interstitial fluid. Our discovery, in 1996 [23], that dorsal root ganglia, which house the cell bodies of perivascular sensory nerves, and perivascular sensory nerves themselves express a receptor for extracellular Ca2+, provides a possible mechanism. The dorsal root ganglion Ca2+ receptor is homologous with the Ca2+ sensing receptor that was initially described in the parathyroid gland, and has subsequently been localized to a number of tissues including the kidney and brain [25,26]. We have shown that the Ca2+ receptor is present in the perivascular nerve network of a variety of rat tissues, including the mesenteric vasculature, intrarenal arteries and cerebral arteries [27]. These observations have led us to propose the hypothesis that changes in interstitial Ca2+ in these tissues activate the sensory nerve Ca2+ receptor which, through an as yet undefined pathway, results in the release of a transmitter which diffuses to underlying smooth muscle and causes hyperpolarizing vasodilation (Figure 1 and ref. [21]). Figure llustrating how transcellular Ca2+ movement in the kidney cortex could modulate vascular tone. (a) Interlobular artery branching into an afferent arteriole (AA) and filtering unit including glomerulus, efferent arteriole (EA) and adjacent proximal tubule. Ca2+ in the blood is filtered at the level of the glomerulus and enters urine. Significant absorption of Ca2+ from the urine occurs at the level of the proximal tubule and enters the interstitial fluid compartment (Ca2+‐isf), where it would be in contact with nearby blood vessels. (b) An expansion of the area in the rectangle outlined in (a). On the left is an arterial segment containing vascular smooth muscle cells (VSMC) and the surrounding perivascular sensory nerve network. On the right is a proximal tubule. Ca2+ leaving the urine enters the interstitial fluid where it is free to interact with the periadventitial surface of the artery. (c) An expansion of the rectangle outlined in (b). Shown on the left is a section of the artery containing vascular smooth muscle cells (VSMC), while on the right is a sensory nerve. In this model, interstitial Ca2+ is sensed by the Ca2+ receptor, which through an unknown signaling pathway induces the production of a nerve‐derived hyperpolarizing relaxing factor (NDHF). NDHF then diffuses to adjacent vascular smooth muscle cells and after activation of a receptor (R) causes opening of iberiotoxin sensitive potassium–calcium channels which result in hyperpolarizing vasodilation. The net effect of an increase in Ca2+ filtered at the glomerulus would thus be an increase in arterial diameter of small arteries in the renal cortex and an increase in blood flow and washout of interstitial Ca2+. In contrast, a decrease in Ca2+ delivery would reduce sensory nerve dilation and increase vascular resistance. It is now clear that Ca2+, over a range of concentrations that can occur in vivo in the small intestine and the kidney, is able to induce sensory nerve‐dependent vasorelaxation. This vasodilator pathway could serve to link changes in vessel diameter and local resistance with fluctuations in interstitial Ca2+, such as should occur normally in the intestine and kidney after consumption of a Ca2+‐containing diet, in response to certain drugs, i.e. thiazide diuretics, or under select pathological conditions, i.e. hyperparathyroidism. Ca2+‐induced dilation could then modulate local blood flow and, in the kidney, could have long‐term effects on salt and water homeostasis and blood pressure through mechanisms described by Cowley and Roman [28]. This pathway might serve as the long sought after link between Ca2+ intake and vascular function, and, if the sensory nerve Ca2+ receptor proves to be the molecular link, this protein might serve as a molecular target for the development of novel antihypertensive vasodilator compounds with regionally specific actions. Correspondence and offprint requests to: Richard Dean Bukoski, Cardiovascular Disease Research Program, Julius L. Chambers Biomedical Biotechnology Research Institute, North Carolina Central University, Durham, NC 27707, USA. This work was supported by grants HL54901, HL59868 and HL64761 from the National Institutes of Health.