Neuropeptide Y as a stimulator of Na+-dependent Ca2+ efflux from freshly isolated adult rat cardiomyocytes

Several physiological stimuli cause a rise in intracellular Ca²⁺ concentration (Ca²⁺]_i) in cardiomyocytes. This increased Ca²⁺]_i must be restored to physiological resting level to ensure response to further stimuli. In the present study, we examined the effect of neuropeptide Y (NPY), which is secreted from certain adrenergic or non-adrenergic neurons, on Ca²⁺ efflux from freshly isolated, quiescent adult rat cardiomyocytes. The isolated cardiomyocytes were preloaded with ⁴⁵CaCl₂ for 1 h. Then, the fractional release of ⁴⁵Ca²⁺ from the cells was measured. NPY stimulated the efflux of ⁴⁵Ca²⁺ from isolated adult rat cardiomyocytes in a concentration-dependent manner (10^–8 M to 10^–6 M). NPY (10^–6 M)-induced Ca²⁺ efflux was 2.0 ± 0.16% of the total cellular content. The ⁴⁵Ca²⁺ efflux from the cells was also stimulated by Y₁ receptor agonist, Leu³¹, Pro³⁴]NPY, but not by Y₂ receptor agonist, NPY_13–36. The effect of NPY was inhibited by a peptide NPY inhibitor, NPY_18–36 and a non-peptide NPY inhibitor, benextramine to a similar extent. From these results, it is conceivable that the effect of NPY on Ca²⁺ efflux from cardiomyocytes is mediated through Y₁ receptors. It was also observed that NPY caused a rise in Ca²⁺]_i to almost 150 nM. NPY-stimulated ⁴⁵Ca²⁺ efflux was not affected by removal of extracellular Ca²⁺, but was dependent on the presence of extracellular Na⁺. Moreover, NPY caused a ²²Na⁺ influx into the cells of about 1.6-fold over the basal value which was inhibited by amiloride and 5-(N,N-dimethyl)-amiloride, known Na⁺/Ca²⁺ exchange inhibitors. In addition, isoproterenol also caused ⁴⁵Ca²⁺ efflux from the cells and which was enhanced by the addition of NPY. These results suggest that NPY stimulates extracellular Na⁺-dependent ⁴⁵Ca²⁺ efflux from freshly isolated adult rat cardiomyocytes, probably through its stimulatory effect on plasma membrane Y₁ receptors with which NPY may couple during Na⁺/Ca²⁺ exchange. Received: 21 May 1997 / Accepted: 26 August 1997