Phasic secretion of newly synthesized atrial natriuretic factor from unstimulated atrial myocytes in culture

We have examined kinetics and composition of newly synthesized proteins secreted from cultured atrial myocytes from adult rats. Under unstimulated conditions, noncontracting cultured atrial myocytes, which were pulse- labeled for 10 minutes with [³⁵S]methionine, rapidly released a considerable portion of newly synthesized atrial natriuretic factor (ANF) in a phasic secretion with a peak at 40 to 80 minutes of chase time. The phasic secretion almost ceased after 80 minutes of chase, after which relatively slow release of the hormone was observed. The ability to stimulate the phasic secretion with secretagogues and a marked resemblance of the radiochemical composition of released proteins in the unstimulated phasic secretion to that in stimulated secretion suggest that the proteins discharged from the cells during the phasic secretion might be derived from secretory granules. Examination of the quantitative change of intracellular ANF showed that ≃60% of newly synthesized labeled ANF was still retained in the cells after the termination of the phasic secretion, indicating that the termination of the phasic secretion was not due to depletion of the labeled protein in the cells. These results suggest that a proportion of newly synthesized ANF was rapidly released from the unstimulated atrial myocytes via a secretory route that shares certain features with both the regulated and the constitutive secretory pathway and that a part of newly synthesized ANF is processed for rapid release while the remainder is destined for slow release or storage within the cells.