ABSTRACT
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.
ABSTRACT
Flocalin (FLO) is a new ATP-sensitive K(+) (KATP) channel opener (KCO) derived from pinacidil (PIN) by adding fluorine group to the drug's structure. FLO acts as a potent cardioprotector against ischemia-reperfusion damage in isolated heart and whole animal models primarily via activating cardiac-specific Kir6.2/SUR2A KATP channels. Given that FLO also confers relaxation on several types of smooth muscles and can partially inhibit L-type Ca(2+) channels, in this study, we asked what is the mechanism of FLO action in bladder detrusor smooth muscle (DSM). The actions of FLO and PIN on contractility of rat and guinea pig DSM strips and membrane currents of isolated DSM cells were compared by tensiometry and patch clamp. Kir6 and SUR subunit expression in rat DSM was assayed by reverse transcription PCR (RT-PCR). In contrast to PIN (10 µM), FLO (10 µM) did not produce glibenclamide-sensitive DSM strips' relaxation and inhibition of spontaneous and electrically evoked contractions. However, FLO, but not PIN, inhibited contractions evoked by high K(+) depolarization. FLO (40 µM) did not change the level of isolated DSM cell's background K(+) current, but suppressed by 20 % L-type Ca(2+) current. Determining various Kir6 and SUR messenger RNA (mRNA) expressions in rat DSM by RT-PCR indicated that dominant KATP channel in rat DSM is of vascular type involving association of Kir6.1 and SUR2B subunits. Myorelaxant effects of FLO in bladder DSM are explained by partial blockade of L-type Ca(2+) channel-mediated Ca(2+) influx rather than by hyperpolarization associated with increased K(+) permeability. Thus, insertion of fluorine group in PIN's structure made the drug more discriminative between Kir6.2/SUR2A cardiac- and Kir6.1/SUR2B vascular-type KATP channels and rendered it partial L-type Ca(2+) channel-blocking potency.
