Alterations in the secretion of atrial natriuretic factor in atria from aged rats

We measured plasma atrial natriuretic factor levels and atrial natriuretic factor secretion by isolated left atria from aging rats to determine the secretory response to stretch and adrenergic stimulation. Systolic arterial pressure and right atrial pressure were measured in vivo. Twenty-four hours later, atria were removed and studied in vitro in a perifusion system. After removal, stabilization at 0.7 g tension, and equilibration for 65 minutes, atria were stretched by increasing external tension for 20 minutes. After reequilibration atria were perifused with phenylephrine, 10(-5) M, for an additional 30 minutes. Right atrial pressure was not different between young (3 months) and aged (16-24 months) rats. Aged rats had higher plasma atrial natriuretic factor levels (52 +/- 8 versus 21 +/- 6 pmol/l; p less than 0.05) than young rats. Basal atrial natriuretic factor secretory rate in vitro was greater in atria from aged rats than young rats (875 +/- 35 versus 402 +/- 22 pg/min; p less than 0.05). Atria from aged rats had an increased response to phenylephrine compared with young rats (1,687 +/- 143 versus 788 +/- 113 pg/min; p less than 0.05) when means were adjusted for basal secretory rate. The secretory response to stretch was less than that of young rats (673 +/- 37 versus 773 +/- 27 pg/min), although this difference was not significant (p = 0.07). Atrial natriuretic factor secretion in response to adrenergic stimulation is increased with aging, and these secretory responses may contribute to increased plasma levels that occur during aging. In contrast to increased adrenergic responses, atrial natriuretic factor secretion after external stretch is not increased in aging rats.