Regulation Of Venous Tone Research Articles

Requirement for Functional BK Channels in Maintaining Oscillation in Venomotor Tone Revealed by Species Differences in Expression of the β1 Accessory Subunits

We determined the possible role of large-conductance Ca2+-activated K (BK) channels in regulation of venous tone in small capacitance veins and blood pressure. In rat mesenteric venous smooth muscle cells (MV SMC), BK channel α- and β1-subunits were coexpressed, unitary BK currents were detected, and single-channel currents were sensitive to voltage and [Ca2+]i. Rat MV SMCs displayed Ca sparks and iberiotoxin-sensitive spontaneous transient outward currents. Under resting conditions in vitro, rat MV exhibited nifedipine-sensitive spontaneous oscillatory constrictions. Blockade of BK channels by paxilline and Ca2+ sparks by ryanodine constricted rat MV. Nifedipine caused venodilation and blocked paxilline-induced, KCl-induced (20 mM), and BayK8644-induced contraction. Acute inhibition of BK channels with iberiotoxin in vivo increased blood pressure and reduced venous capacitance, measured as an increase in mean circulatory filling pressure in conscious rats. BK channel α-subunits and L-type Ca2+ channel α1-C subunits are expressed in murine MV. However, these channels are not functional because murine MV lack nifedipine-sensitive basal tone and rhythmic constrictions. Murine MV were also insensitive to paxilline, ryanodine, KCl, and BayK8644, consistent with our previous studies showing that murine MV do not have BK β1-subunits. These data show that not only there are species-dependent properties in ion channel control of venomotor tone but also BK channels are required for rhythmic oscillations in venous tone.

Role of Baroreceptors in the Zone of Vertebral Arteries in the Reflex Regulation of Venous Tone in the Splanchnic Basin

Pressure elevation in the reflexogenic zone of vertebral arteries was accompanied by a decrease in the tone of splanchnic veins, reduction of blood pressure, and suppression of external respiration. An opposite response of the cardiorespiratory functional system was observed under conditions of low baseline pressure in the vascular zone. Our findings and results of previous physiological and morphological studies indicate that the capacitance vessels play an important role in cardiorespiratory reactions.

Endothelins 1 and 3 and big endothelin-1 contract isolated human placental veins.

Experiments were designed to investigate the reactivity of vascular smooth muscle to endothelins (ETs) in veins taken from human placentas immediately after delivery. The placental veins were cut into rings and suspended between two stirrups in conventional organ chambers (filled with aerated, modified Krebs-Ringer bicarbonate solution) for isometric recording of tension. ET-1 and ET-3 caused concentration-dependent contractions of the isolated human placental veins. The responses induced by ET-1 were greater than those evoked by ET-3 and were not significantly affected by BQ-123, a selective inhibitor of ETA receptors. Contractions to big ET-1 were obtained in rings both with and without endothelium; they were inhibited by phosphoramidon, an inhibitor of endothelin-converting enzyme. These findings indicate that the conversion of the precursor of ET-1 can occur in human placental veins. The receptors mediating the contraction of human placental veins to endothelins do not belong to the ETA subtype; the response to the peptides is probably mediated in part by an uncharacterized ET-receptor subtype and in part by ETB receptors. The output of big ET-1 in the vascular wall or from surrounding tissues in the placenta could be involved in the regulation of venous tone in this organ.