Mechanical and neuroendocrine regulation of the endocrine heart

The cardiac natriuretic peptides (NP) -- atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) -- are polypeptide hormones produced by cardiocytes in the atria of mammals. ANF and BNP are continuously released from the heart, but appropriate mechanical or neuroendocrine stimuli increase their rate of release with or without a concomitant increase in synthesis. The results of our investigations lead us to propose that the endocrine response of the heart to pressure or volume load varies in relation to whether the challenge is acute, subacute or chronic. The acute response to stretch is based on a phenomenon referred to as "stretch-secretion coupling" which results in enhanced secretion of NP stored in the atria. NP release following stretch is made at the expense of a depletable NP pool with no apparent effect on synthesis. The stimulation of NP production that is seen during mineralcorticoid escape is referred to as "subacute" and is characterized by stimulation of atrial ANF and BNP gene transcription secondary to volume overload in which plasma ANF, but not plasma BNP, is significantly elevated. With chronic stimulation, as seen in DOCA-salt treatment at the hypertensive stage, activation of the cardiac fetal program in ventricle is seen together with a stimulation of ANF and BNP production in both atria and ventricles. However, the activation of NP gene expression in the atria is not necessarily associated with fetal isogene expression even though the ventricular hypertrophic process is characterized by the expression of fetal isogenes, including ANF and BNP, that are normally expressed in the fetal ventricle. It seems likely that the acute stimulation of NP release is based on an electromechanical coupling. However, protracted stimulation of release is seen in situations in which profound neuroendocrine changes have taken place, thus suggesting that the primary stimulus for chronically enhanced NP gene expression and NP release is based on changes in the hormonal environment of the atrial cardiocyte. It is concluded that the endocrine heart responds to changes in hemodynamic load with specific changes in translational, post-translational and storage processes for ANF and BNP following acute or chronic stimulation. As a result, plasma levels of ANF and BNP may be used as indicators of the degree of atrial hemodynamic overload and ventricular hypertrophy, respectively. It may be advanced that the endocrine heart differentiates and responds to different hemodynamic challenges in either acute or chronic conditions with specific changes in transcription, translation, post-translational processing, storage, and release of ANF and BNP. We propose that this differentiation is part of the reason for the heart to produce two hormones with similar spectra of activity. This paradigm warrants further investigation.