ISOPROTERENOL INDUCES RELEASE OF ATRIAL-NATRIURETIC-PEPTIDE FROM RAT ATRIUM INVITRO

To investigate the mechanism underlying the release of atrial natriuretic peptide (ANP) in in vitro condition, isolated, superfused rat atria were subjected to adrenergic, chronotropic, and mechanical stimulation. First administration of isoproterenol (Iso; either 10(-9) or 10(-6) M) caused a release of ANP, which was transient. Subsequent increments in concentration of Iso always resulted in a much lower release of ANP, despite the increased effects on the mechanical function of the atria. Stretching of the atria resulted in a transient release of ANP. Subsequent increments in stretching were followed by decreasing release of ANP. The total store of ANP in atrial tissue after Iso and stretching was not measurably depleted. Pacing the atria with increasing frequency did not induce release of ANP. Depolarization with 40 mM KCl abolished the release of ANP in response to Iso but not the release induced by stretch. In the presence of low external Ca2+, which abolished mechanical activity, both Iso and stretch could still induce release of ANP. Propranolol abolished the release of ANP by Iso but not that induced by stretching. Prazosin did not affect the release by either stretch or Iso. Stretching the atria 20 min after administration of Iso did not cause any further release of ANP. On the other hand, adding Iso 20 min after stretching induced a release of ANP. It is concluded that Iso and stretch cause a transient release from isolated strips of atria. The amount of ANP released is not related to the dose of Iso or to the load applied. Mechanisms involved in the release mediated by the two stimuli are different. Release of ANP by Iso requires a membrane potential; it is not mediated by the mechanical effect of the drug but by beta-adrenergic mechanisms. Release induced by stretching does not require a membrane potential, is independent of beta- or alpha-blockade, and seems to be mediated purely by mechanical stress.