Age-related differences in isolated rat sinus node function.

In order to rule out age-related differences in the sinus node (SN) function, the effects of different substances were tested in SN isolated preparations from adult (4-5-month-old) and old (24-25-month-old) rats. No difference was seen in the in vivo heart rate between adult and old rats, whereas sinus node rate (SNR) was significantly lower in the old rather than in the adult preparations. The effects of acetylcholine (10(-9)-10(-5) M) were similar between the two groups of preparations. The response to adenosine (10(-9)-10(-4) M) was significantly higher in the adult than in the old preparations (p less than 0.05). No significant difference was noted in the response to isoproterenol (10(-10)-10(-6) M). High calcium (5.4-8.1 mM) caused an increase of the SNR that was significantly greater in the old than in the adult preparations (p less than 0.005). In conclusion, our results show an age-related decrease in the isolated SNR that does not seem to be mediated by different responses to neuromediators but is probably due to an alteration of the intrinsic activity of the SN cells.