The Calcium Sensing Receptor Activation by Neomycin Decreases the Bicarbonate Flux in Renal Proximal Tubules of Wistar Rats

Abstract

In the parathyroid gland, high extracellular Ca2+ activates Calcium Sensing Receptor (CaSR) and suppresses PTH secretion.In the renal proximal tubule (PT), PTH inhibits NHE3 and CaSR expression, indicating that NHE3 is regulated by Ca2+ homeostasis. Since 20% of the filtered Ca2+ is reabsorbed via transcellular, CaSR activation may regulate NHE3 activity through cell Ca2+ entry. The aim of this study was to determine the effect of CaSR activation and cell Ca2+ entry on PT bicarbonate reabsorption (JHCO3-), via stationary microperfusion. To show the role of CaSR in JHCO3-, PT were perfused with a 24mM HCO3- and 1mM Ca2+ solution (CTRL) in the presence of the CaSR agonist Neomycin (Neo, 200µM). Neo decreased JHCO3- by 27.86% (vs CTRL), showing that CaSR activation may inhibit NHE3. The addition of the specific CaSR antagonist NPS2143 (NPS, 2µM) to Neo was able to abolish its effect, showing that Neo-dependent JHCO3- inhibition was due to CaSR activation. To determine if cell Ca2+ entry was involved in this effect, the non-specific TRP antagonist ruthenium red (RR, 5µM) or the specific L-type Ca2+ channel blocker Nifedipine (Nif, 200µM) (±Neo) were used. RR alone inhibited JHCO3-, showing that TRP channels are endogenously important to maintain JHCO3-. Interestingly, RR abolished the Neo-dependent JHCO3- inhibition indicating that TRP channels are involved in the Neo effect in an opposite way as its physiological role. Perfusion of Nif alone had no effect on JHCO3- and was unable to abolish Neo-dependent JHCO3- inhibition, showing that L-type Ca2+ channels are not involved in this effect. These data show that CaSR-dependent cell Ca2+ entry via TRP channels inhibit JHCO3-. Financial support: FAPESP; CNPq