Submitted on April 18, 2005
Accepted on September 21, 2005
Modulation by Dietary Sodium Intake of Melanocortin 3 Receptor mRNA and Protein Abundance in the Rat Kidney
Xi-Ping Ni1, Aditi Bhargava1, David Pearce1, and Michael H Humphreys1*
1 Division of Nephrology, San Francisco General Hospital and Department of Medicine, University of California San Francisco, San Francisco, CA, USA
* To whom correspondence should be addressed. E-mail: mhumphreys{at}medsfgh.ucsf.edu.
-Melanocyte stimulating hormone (-MSH) is a circulating natriuretic peptide hormone derived from proopiomelanocortin (POMC); its concentration in plasma, and pituitary POMC mRNA abundance, increase in rats ingesting a high sodium diet (HSD, 8% NaCl) compared to a low sodium diet (LSD, 0.07 % NaCl). Reverse transcription-polymerase chain reaction amplification (RT-PCR) of rat kidney RNA demonstrated reaction products of the expected size in both cortex and medulla for MC3-R, MC4-R, and MC5-R mRNA; no signal for MC1-R or MC2-R was detected. Relative to 
-actin or cyclophilin, abundance of the three receptor transcripts after 1 week of the LSD was approximately equal in both cortex and medulla. After one week of the HSD, mRNA abundance of MC4-R and MC5-R was unchanged while that of MC3-R in medulla more than doubled, the ratio of MC3-R/-actin signal increasing from 0.38 ± 0.04 on LSD to 0.84 ± 0.04 on HSD (p <.001). No significant increase occurred in cortex. The increase in MC3-R expression induced by dietary sodium was observed in inner medullary collecting duct (IMCD) cells isolated from HSD rat kidneys, suggesting that these cells were the major site of receptor expression in the medulla. Immunoblots of whole medullary and IMCD cell homogenates detected MC3-R immunoreactive protein; its expression was twice as great in samples from HSD vs LSD rat kidneys, paralleling the increase in MC3-R mRNA abundance on the HSD. No changes in MC4-R or MC5-R protein expression were observed. Incubation of IMCD cell suspensions with increasing concentrations of 2-MSH led to increased cAMP accumulation, with values from rats on the HSD being roughly double the values from LSD rats. Intrarenal infusion of 2-MSH (500 fmol/min) increased sodium and cAMP excretion from the infused but not contralateral kidney of HSD rats while having no effect in LSD rats. These data show that MC3-R is expressed in rat IMCD cells in a manner modulated by dietary sodium intake. Since MC3-R is the receptor with which -MSH interacts, our findings suggest the existence of a sodium-regulating system, activated in response to a HSD, which increases urinary sodium excretion to balance the high sodium intake.
