Halothane inhibition of acetylcholine-induced relaxation in rat mesenteric artery and aorta

Contribution of K(+) channels to endothelium-derived hypolarization-induced renal vasodilation in rats in vivo and in vitro

We investigated the mechanisms behind the endothelial-derived hyperpolarization (EDH)-induced renal vasodilation in vivo and in vitro in rats. We assessed the role of Ca(2+)-activated K(+) channels and whether K(+) released from the endothelial cells activates inward rectifier K(+) (Kir) channels and/or the Na(+)/K(+)-ATPase. Also, involvement of renal myoendothelial gap junctions was evaluated in vitro. Isometric tension in rat renal interlobar arteries was measured using a wire myograph. Renal blood flow was measured in isoflurane anesthetized rats. The EDH response was defined as the ACh-induced vasodilation assessed after inhibition of nitric oxide synthase and cyclooxygenase using L-NAME and indomethacin, respectively. After inhibition of small conductance Ca(2+)-activated K(+) channels (SKCa) and intermediate conductance Ca(2+)-activated K(+) channels (IKCa) (by apamin and TRAM-34, respectively), the EDH response in vitro was strongly attenuated whereas the EDH response in vivo was not significantly reduced. Inhibition of Kir channels and Na(+)/K(+)-ATPases (by ouabain and Ba(2+), respectively) significantly attenuated renal vasorelaxation in vitro but did not affect the response in vivo. Inhibition of gap junctions in vitro using carbenoxolone or 18α-glycyrrhetinic acid significantly reduced the endothelial-derived hyperpolarization-induced vasorelaxation. We conclude that SKCa and IKCa channels are important for EDH-induced renal vasorelaxation in vitro. Activation of Kir channels and Na(+)/K(+)-ATPases plays a significant role in the renal vascular EDH response in vitro but not in vivo. The renal EDH response in vivo is complex and may consist of several overlapping mechanisms some of which remain obscure.

Effects of General Anesthetics on Regulation of the Peripheral Vasculature

The heart is a passively filling pump in a circulatory system that is connected in series with distensible blood vessels. Therefore, systemic blood pressure and tissue perfusion depend upon adequate peripheral vascular tone as well as myocardial function. Likewise, pharmacologic agents that alter circulatory stability can affect one or both of these components. The generalized depressor effects of general anesthetics have been well known clinically for over 50 years. Moreover, there are many similarities in basic cellular regulatory mechanisms among the different tissue types, and general anesthetics are well known to distribute freely among the perfusion-rich tissues (eg, central nervous system, cardiovascular system, and renal system). Therefore, it is likely that the hemodynamic depression resulting from the systemic administration of anesthetics results from actions on regulatory mechanisms of the peripheral vasculature as well as on the heart. The peripheral vasculature is regulated by extrinsic neural, endothelial, and humoral mechanisms, which interact with each other as well as with intrinsic membrane and intracellular systems within the vascular smooth muscle cell. Different general anesthetics have been found to act on specific mechanisms at each of these levels. However, the large number and complexity of these known mechanisms, as well as the many anesthetic agents, has made it extremely difficult to determine which are significant in terms of the meaningful mechanisms that are responsible for anesthetic action, major side effects, or both. Current knowledge about the effects of general anesthetics on both the extrinsic intrinsic regulatory mechanisms of peripheral vascular control is reviewed.

Desflurane, compared to halothane, augments phenylephrine-induced contraction in isolated rat aorta smooth muscle

PURPOSE

The mechanism responsible for the mediation of hypertension in response to increased desflurane levels is unclear. This study compared the effect of desflurane and halothane on phenylephrine (PE)-induced contraction in rat aorta ring and the effect of desflurane in the presence and absence of nitric oxide (NO) synthase activity.

METHODS

Endothelium-free rat aorta rings were exposed serially to 10(-7) M, 10(-6) M and 10(-5) M PE alone and subsequently in the presence of 2 MAC desflurane and halothane. Secondly, endothelium-free preparations were exposed to 10(-6) M PE serially in the presence of 0, 1, 2 and 3 MAC desflurane and halothane. Thirdly, using an endothelium-intact preparation, the effect of desflurane on PE-induced contraction was examined, in the presence or absence of NG-nitro-L-arginine (L-NNA), an inhibitor of constitutive and inducible NO synthase.

RESULTS

Contraction amplitudes secondary to 10(-6) and 10(-5) M PE in endothelium-free preparations were increased by 74% and 36% respectively (P <0.05) in the presence of 2 MAC desflurane compared to controls. In endothelium-free preparations, contraction amplitudes secondary to 10(-6) M PE were increased in the presence of 1 and 2 MAC desflurane by 32% and 18% respectively (P <0.05) and reduced by 16% in the presence of 3 MAC halothane (P <0.05). In endothelium-intact preparations an expected absolute increase in contraction amplitude occurred in the presence of L-NNA but the desflurane effect was detectable both in the presence and absence of L-NNA.

CONCLUSION

Our results suggest that desflurane may have a local vasoconstrictive effect independent of endothelium and NO synthase activity. The mechanism remains to be determined.

Inhibition of the human intermediate conductance Ca(2+)-activated K(+) channel, hIK1, by volatile anesthetics

Ca(2+)-activated K(+) channels (K(Ca)) regulate a wide variety of cellular functions by coupling intracellular Ca(2+) concentration to membrane potential. There are three major groups of K(Ca) classified by their unit conductances: large (BK), intermediate (IK), and small (SK) conductance of channels. BK channel is gated by combined influences of Ca(2+) and voltage, while IK and SK channels are gated solely by Ca(2+). Volatile anesthetics inhibit BK channel activity by interfering with the Ca(2+) gating mechanism. However, the effects of anesthetics on IK and SK channels are unknown. Using cloned IK and SK channels, hIK1 and hSK1-3, respectively, we found that the currents of hIK1 were inhibited rapidly and reversibly by volatile anesthetics, whereas those of SK channels were not affected. The IC(50) values of the volatile anesthetics, halothane, sevoflurane, enflurane, and isoflurane for hIK1 inhibition were 0.69, 0.42, 1.01 and 1.03 mM, respectively, and were in the clinically used concentration range. In contrast to BK channel, halothane inhibition of hIK1 currents was independent of Ca(2+) concentration, suggesting that Ca(2+) gating mechanism is not involved. These results demonstrate that volatile anesthetics, such as halothane, enflurane, isoflurane, and sevoflurane, affect BK, IK, and SK channels in distinct ways.