A possible mechanism of inotropic action of prolactin on rat heart

Effects of prolactin on ventricular myocyte shortening and calcium transport in the streptozotocin-induced diabetic rat

The physiological role of prolactin (PRL) in the heart, and in particular the diabetic heart, are largely unknown. The effects of PRL on ventricular myocyte shortening and Ca²⁺ transport in the streptozotocin (STZ) – induced diabetic and in age-matched control rats were investigated. PRL receptor protein, myocyte shortening, intracellular [Ca²⁺], L-type Ca²⁺ current were measured by Western blot, cell imaging, fluorescence photometry and whole-cell patch-clamp techniques, respectively. Compared to normal Tyrode solution (NT), PRL (50 ng/ml) significantly (p < 0.05) increased the amplitude of shortening in myocytes from control (7.43 ± 0.38 vs. 9.68 ± 0.46 %) and diabetic (6.57 ± 0.24 vs. 8.91 ± 0.44 %) heart (n = 44–49 cells). Compared to NT, PRL (50 ng/ml) significantly increased the amplitude of Ca²⁺ transients in myocytes from control (0.084 ± 0.004 vs. 0.115 ± 0.007 Fura-2 ratio units) and diabetic (0.087 ± 0.007 vs. 0.112 ± 0.006 Fura-2 ratio units) heart (n = 36–50 cells). PRL did not significantly alter the amplitude of caffeine-evoked Ca²⁺ transients however, PRL significantly increased the fractional release of Ca²⁺ in myocytes from control (21 %) and diabetic (14 %) and heart. The rate of Ca²⁺ transient recovery following PRL treatment was significantly increased in myocytes from diabetic and control heart. Amplitude of L-type Ca²⁺ current was not significantly altered by diabetes or by PRL. PRL increased the amplitude of shortening and Ca²⁺ transients in myocytes from control and diabetic heart. Increased fractional release of sarcoplasmic reticulum Ca²⁺ may partly underlie the positive inotropic effects of PRL in ventricular myocytes from control and STZ-induced diabetic rat.

The prolactin family hormones regulate vascular tone through NO and prostacyclin production in isolated rat aortic rings

AIM

Prolactin family hormones include growth hormone, placental lactogen and prolactin, which are able to regulate angiogenesis via NO and prostaglandins. However, their effects on vascular tone are not fully understood. The aim of this study was to evaluate the effects of prolactin family hormones on rat vascular tone in vitro.

METHODS

Aortic rings were prepared from adult male rats and precontracted with phenylephrine, then treated with the hormones and drugs. The tension was measured with isometric force displacement transducer connected to a polygraph. NO production and prostacyclin release in physiological solution was determined. Cultured rat aortic endothelial cells (RAECs) were treated with the hormones and drugs, and the phosphorylation of eNOS at serine 1177 was assessed using Western bolt analysis.

RESULTS

Administration of growth hormone or placental lactogen (0.01-100 nmol/L) induced endothelium-dependent vasodilation. Both the hormones significantly increased the phosphorylation of eNOS in RAECs and NO level in physiological solution. Preincubation with L-NAME blocked growth hormone- or placental lactogen-induced vasodilation and NO production. Preincubation with an antibody against growth hormone receptors blocked growth hormone- and placental lactogen-induced vasodilation. Addition of a single dose of prolactin (0.01 nmol/L) induced sustained vessel relaxation, whereas multiple doses of prolactin induced a biphasic contraction-relaxation effect. The vascular effects of prolactin depended on endothelium. Prolactin significantly increased the level of prostacyclin I2 in physiological solution. Preincubation with indomethacin or an antibody against prolactin receptors blocked prolactin-induced vasodilation.

CONCLUSION

The prolactin family hormones regulate rat vascular tone, selectively promoting either relaxation or contraction of vascular smooth muscle via activation of either growth hormone receptors or prolactin receptors within the endothelium.