Central Calcineurin Plays a Role in Skeletal Muscle Reflex Overactivity Induced by High Dietary Phosphate Intake in Rats

Abstract

Inorganic phosphate (Pi) has emerged as an important dietary risk factor for hypertension. Our laboratory has previously shown that high dietary Pi intake augments blood pressure and sympathetic nerve activity during muscle contraction in otherwise normal rats primarily due to an overactive exercise pressor reflex (EPR). However, the mechanism underlying the abnormal responsiveness induced by dietary Pi excess remains poorly understood. Evidence suggests that a high Pi diet-induced increase in the phosphaturic hormone fibroblast growth factor 23 activates calcineurin in cardiomyocytes, potentially contributing to left ventricular hypertrophy. Calcineurin has been shown to exert direct central sympathoexcitatory action in rodents but its role in blood pressure control during dietary Pi excess is unknown. Accordingly, we conducted studies to examine the role of brain calcineurin in generating high Pi diet-induced EPR overactivation in the Sprague-Dawley rats fed either a normal 0.6% Pi diet (NP) or a high 1.2% Pi diet (HP) for 12 weeks. In decerebrate NP and HP animals, we measured mean arterial pressure (MAP) response to hindlimb muscle contraction (i.e. activation of the EPR) or hindlimb intra-arterial capsaicin injection (i.e. activation of the metabolically-sensitive component of the EPR known as the metaboreflex) before and after intracerebroventricular (ICV) administration of a calcineurin inhibitor cyclosporin A. We also assessed levels of regulator of calcineurin 1.4 (Rcan1.4), a protein that is upregulated in response to increased calcineurin activity in the brainstem. We found that brainstem Rcan1.4 expression was significantly increased by 60% in HP rats compared to NP rats (P<0.05), suggesting activation of calcineurin. Calcineurin expression in the brainstem was also greater in the HP group than in the NP group (9.2±0.6 vs. 1.0±0.6 arbitrary unit, p<0.05). ICV injection of cyclosporin A significantly attenuated the MAP (∆=28±13 vs. 7±3 mmHg, P<0.05) response to EPR activation in HP animals, but did not alter the response in NP animals (∆MAP=12±6 vs. 9±2 mmHg, P>0.05). Similarly, the potentiated MAP response to stimulation of the metaboreflex was significantly decreased by ICV cyclosporin A in HP rats (∆=49±7 vs. 25±5 mmHg, P<0.05), but remained unchanged in NP rats (∆MAP=26±3 vs. 20±8 mmHg, P>0.05). In conclusion, the results implicate brain calcineurin activity in the abnormal skeletal muscle reflex-mediated hypertensive responsiveness induced by a high Pi diet.