Submitted on November 25, 2002
Accepted on March 12, 2003
Age-related functional effects linked to phosphatase activity in ventricular myocytes
Elizabeth M. Grey1, Chun K. Chan1, Yi Chen1, and Polly A. Hofmann1*
1 Department of Physiology, University of Tennessee, Memphis, TN, USA
* To whom correspondence should be addressed. E-mail: phofmann{at}physio1.utmem.edu.
Conflicting reports exist regarding the influence of 
-adrenergic stimulation on maximum velocity of shortening (Vmax) in ventricular myocytes. This may be due to an unrecognized effect of maturation. In the present study, the effects of -adrenergic receptor stimulation on myocytes from hearts of juvenile non-bred, and young adult retired breeder female rats were compared. Ventricular myocytes from young adults had a -adrenergic-dependent increase in Vmax and Ca2+ dependent actomyosin ATPase that was not observed in myocytes from juveniles. Myocytes from young adults had both an increase in -myosin heavy chain (MHC), and a higher basal ser/thr phosphatase activity as compared to juvenile rats. Additional studies established moderate increases in -MHC induced by hypothyroidism do not confer myocardial -adrenergic responsiveness, while inhibition of the higher phosphatase activity in myocytes from young adults blocks the age-dependent, -adrenergic - induced increase in crossbridge cycling rates. We propose the higher phosphatase activity of myocytes from young adults, as compared to juveniles, allows for a greater functional response of the myocardium to -adrenergic stimulation.
