The rat thymus--a site of atrial natriuretic peptide synthesis.

The atrium of the heart has been demonstrated to represent the major site of synthesis of atrial natriuretic peptide (ANP), a potent natriuretic, diuretic and vasoactive hormone. Our recent studies revealed ANP-like material outside the heart, namely, in lymphoid follicles of the intestine and in the thymus, and now we report data demonstrating the thymus as a site of synthesis for ANP. The experimental evidence is as follows: firstly, the immunoreactive material detected corresponds chromatographically with the precursor of ANP. Secondly, the thymus contains mRNA for ANP. Thirdly, immunohistochemistry locates ANP-like material to cortical thymocytes with particularly dense staining in the subcapsular areas of the thymus. Interestingly, both ANP-like material and the mRNA coding for ANP were expressed to a larger extent in newborn rats as compared to adult animals, suggesting that ANP may be involved in the development and/or function of T-cells.