Differences in alpha2-adrenoceptor modulation of calcium channels in vascular smooth muscle cells of male and female rats

Diazepam augments gender differences in cutaneous LD flux response to local cooling

Cutaneous vasoconstriction in response to local cooling is normally greater in females than in males. Cold induces amplification of alpha2-adrenoceptor affinity for norepinephrine and increases reflex sympathetic thermoregulatory output. Benzodiazepines are drugs with very well-known binding to the central and peripheral benzodiazepine receptors. Besides these effects they decrease sympathetic output and as it was shown in the last decade they act synergistically with alpha-adrenoceptors. In the present study we tested the hypothesis that the benzodiazepine diazepam interacts with an alpha-adrenoceptor mechanism at the level of microcirculation. We measured laser-Doppler blood flux changes provoked by local cooling before and after oral application of a low dose of diazepam (5 mg) in 9 healthy males and in 11 healthy females with regular menstrual cycles. The results of our experiments show that in females there is a significant reduction (ANOVA, p < 0.05) in laser-Doppler flux during the first four minutes of cooling after taking of diazepam. In males, there is no significant difference in the responses to cold before and after an application of diazepam. Our results suggest that diazepam, in addition to its well-known effect on BZ receptors may also interact with alpha2C-adrenoceptors in the vessel wall during local cooling.

The role of α1- and α2-adrenoceptors in gender differences in cutaneous LD flux response to local cooling

The present study was designed to assess the hypothesis that alpha2-adrenergic response plays a predominant role in sex differences in cutaneous microvascular response to cold. For this purpose, we studied the effect of the selective alpha1-antagonist prazosin (1 microl of 1.2 mM solution) and the alpha2-antagonist yohimbine (1 microl of 12 mM solution) microinjected into the skin area where the laser-Doppler (LD) flux response to local cooling was measured in healthy male and female subjects. Multiple regression analysis showed correlation between LD flux response at the site of local cooling (direct response) and gender (P = 0.039). The LD flux decrease was smaller in males. The application of the yohimbine significantly diminished the LD flux response at the site of local cooling in females but not in males (P < 0.05). In contrast, the injection of prazosin did not significantly affect the response. These findings strongly support the suggestion of a decisive role of alpha2-adrenoceptors in cold-induced gender difference at the level of cutaneous microvasculature at the site of local cooling. Multiple regression analysis of LD flux response to cold exposure at the site distant to cooling (indirect response) also confirmed correlation between LD flux response and gender (P = 0.022). The LD flux decrease was smaller in males. The alpha1-antagonist abolished the LD flux response to cold exposure (P = 0.000). These results corroborate the importance of alpha1-adrenoceptors in skin microcirculatory response to indirect local cooling mediated by the sympathetic vascular reflex.

Characterization of voltage-gated calcium currents in freshly isolated smooth muscle cells from rat tail main artery

The aim of the present study was to characterize voltage-gated Ca2+ currents in smooth muscle cells freshly isolated from rat tail main artery in the presence of 5 mmol L(-1) external Ca2+. Calcium currents were identified on the basis of their voltage dependencies and sensitivity to nifedipine, Ni2+ and cinnarizine. In the majority of the cells studied, T- and L-type currents were observed, while the remaining cells showed predominantly L-type currents. In the latter group of cells, holding potential change from -50 to either -70 or -90 mV increased the corresponding inward current amplitude while its voltage activation threshold remained unchanged. The steady state inactivation of L-type Ca2+ channels showed half-maximal inactivation at -38 mV. A Ca2+-dependent inactivation was also evident. Nifedipine (3 micromol L(-1)) blocked L-type but not T-type Ca2+ currents. Ni2+ (50 micromol L(-1)) as well as cinnarizine (1 micromol L(-1)) suppressed the nifedipine-resistant, T-type component of the currents. At higher concentrations, both Ni2+ (0.3-1 mmol L(-1)) and cinnarizine (10 micromol L(-1)) blocked the net inward current. Replacement of Ca2+ with 10 mmol L(-)1 Ba2+ significantly increased the amplitude of L-type Ca2+ currents. These results demonstrate that smooth muscle cells freshly isolated from rat tail main artery may be divided into two populations, one expressing both L- and T-type and the other only L-type Ca2+ channels. Furthermore, this report shows that in arterial smooth muscle cells cinnarizine potently inhibited T-type currents at low concentrations (1 micromol L(-1)) but also blocked L-type Ca2+ currents at higher concentrations (10 micromol L(-1)).

Gender difference in levels of alpha2-adrenoceptor mRNA in the rat tail artery

To investigate the hypothesis that differing mRNA levels underlie gender differences in the contractile response of the rat tail artery, alpha2-adrenoceptor mRNA was measured using in situ hybridization. Messenger RNA for the alpha2A- and alpha2C-adrenoceptor subtypes was found localized to the smooth muscle layer. There was no detectable mRNA present for the alpha2B-adrenoceptor subtype. Levels of alpha2C-adrenoceptor mRNA were greater in female compared to male tail arteries (417 +/- 35 vs. 263 +/- 38 dpm/mg, P = 0.01), while levels of alpha2A-adrenoceptor mRNA were the same in both sexes. Levels of alpha2-adrenoceptor mRNA may parallel levels of functioning protein present in the rat tail artery.