EVIDENCE FOR THE PRESENCE OF SMOOTH-MUSCLE ALPHA-ACTIN WITHIN PERICYTES OF THE RENAL MEDULLA

This study was designed to determine whether smooth muscle alpha-actin mRNA and smooth muscle alpha-actin contractile protein elements were present within the renal medullary pericytes. Extraction of total RNA from microdissected outer medullary descending vasa recta allowed for the detection of smooth muscle alpha-actin mRNA expression using rever se transcription-polymerase chain reaction (RT-PCR). Expression of smo oth muscle alpha-actin was specific to the descending vasa recta and n ot a result of tubular contamination because RT-PCR amplification of t he vasopressin V-2 receptor, which is a specific tubular marker, did n ot occur. To determine the exact cell type(s) that translate the mRNA into protein, eve performed immunohistochemistry on the renal outer an d inner medulla using a monoclonal smooth muscle alpha-actin antibody, whose specificity was determined by immunoblot analysis. Smooth muscl e alpha-actin protein was found selectively within the pericytes surro unding the descending vasa recta from the outer and inner medullary ti ssue sections. This study demonstrates that the pericytes alone that s urround the descending vasa recta within the outer and inner medulla c ontain smooth muscle alpha-actin mRNA and protein and are therefore th e site of the contractile elements that could play a vasomodulatory ro le in the control of renal medullary blood flow and its distribution w ithin the renal medulla.