Constriction velocities of renal afferent and efferent arterioles of mice are not related to SMB expression

Constriction velocities of renal afferent and efferent arterioles of mice are not related to SMB expression. Background Constriction of renal arterioles contributes significantly to the control of perfusion and glomerular filtration. Afferent but not efferent arterioles express smooth muscle myosin heavy chain B (SMB) (with a 5′-insert of seven amino acids). The aim of the present study was to investigate ( 1 ) the constriction characteristics of afferent and efferent arterioles under physiologic load and ( 2 ) whether expression of SMB may causally contribute to these constriction characteristics. Methods We compared constriction parameters [constriction amplitude, maximal rate of constriction velocity ("dc/dt max "), and time to half-maximal constriction (t 1/2 ) of in vitro perfused renal afferent and efferent arterioles of wild-type (smb(+/+)] and homozygous SMB knockout [smb(-/-)] mice upon stimulation with angiotensin II (Ang II) (10 -8 mol/L) and potassium chloride (KCl) (100 mmol/L). SMB expression was investigated by double-labeling immunofluorescence. Results Contraction amplitude and dc/dt max of mouse afferent arterioles upon Ang II stimulation were significantly greater compared to efferent arterioles. However, constriction amplitudes, dc/dt max , and t 1/2 of afferent as well as efferent arterioles upon Ang II stimulation were similar in smb(+/+) and smb(-/-) mice. Constriction amplitudes upon KCl stimulation of afferent arterioles were similar in both smb(+/+) and smb(-/-) mice. Furthermore, KCl-induced dc/dt max and t 1/2 of afferent arterioles were similar in both smb(+/+) and smb(-/-) mice. SMB expression could be detected in afferent but not efferent arterioles in smb(+/+) mice. No SMB expression in either arteriole could be observed in smb(-/-) mice. Conclusion Our results suggest that the presence of different alternatively 5′-spliced smooth muscle-myosin heavy chain (SM-MHC) isoforms does not dominate the different contractile features of physiologically loaded renal afferent or efferent arterioles.