Effects of platelet-activating factor and thromboxane A2 on isolated perfused guinea pig liver.

Lipid mediators, thromboxane A2 (TxA2) and platelet-activating factor (PAF), are potent vasoconstrictors, and have been implicated as mediators of liver diseases, such as ischemic-reperfusion injury. We determined the effects of a TxA2 analogue (U-46619) and PAF on the vascular resistance distribution and liver weight (wt) in isolated guinea pig livers perfused with blood via the portal vein. The sinusoidal pressure was measured by the double occlusion pressure (P(do)), and was used to determine the pre- (R(pre)) and post-sinusoidal (R(post)) resistances. U-46619 and PAF concentration-dependently increased the hepatic total vascular resistance (R(t)). The minimum concentration at which significant vasoconstriction occurs was 0.001 microM for PAF and 0.1 microM for U-46619. Moreover, the concentration of U-46619 required to increase R(t) to the same magnitude is 100 times higher than PAF. Thus, the responsiveness to PAF was greater than that to U-46619. Both agents increased predominantly R(pre) over R(post). U-46619 caused a sustained liver weight loss. In contrast, PAF also caused liver weight loss at lower concentrations, but it produced liver weight gain at higher concentrations (2.5 +/- 0.3 per 10g liver weight at 1 microM PAF), which was caused by substantial post-sinusoidal constriction and increased P(do). In conclusion, both TxA2 and PAF contract predominantly the pre-sinusoidal veins. TxA2 causes liver weight loss, while PAF at high concentrations increases liver weight due to substantial post-sinusoidal constriction in isolated guinea pig livers.

Osmosensitive properties of rapid and slow delayed rectifier K+ currents in guinea-pig heart cells.

1. Changes in cell volume affect a variety of sarcolemmal transport processes in the heart. To study whether osmotically induced cell volume shrinkage has functional consequences for K+ channel activity, guinea-pig cardiac preparations were superfused with hyperosmotic Tyrode's solution (1.2-2-fold normal osmolality). Membrane currents and cell surface dimensions were measured from whole-cell patch-clamped ventricular myocytes and membrane potentials were recorded from isolated ventricular muscles and non-patched myocytes. 2. Hyperosmotic treatment of myocytes quickly (< 3 min to steady state) shrank cell volume (approximately 20% reduction in 1.5-fold hyperosmotic solution) and depressed the delayed rectifier K+ current (IK). Analysis using different activation protocols and a selective inhibitor (5 micro mol/L E4031) indicated that the IK inhibition was due to osmolality and cell volume-dependent changes in the two subtypes of the classical cardiac IK (rapidly activating IKr and slowly activating IKs); 1.5-fold hyperosmotic treatment depressed the amplitudes of IKr and IKs by approximately 30 and 50%, respectively. 3. Superfusion of muscles and myocytes with 1.5-fold hyperosmotic solution lengthened the action potentials by 14-17%. Hyperosmotic treatment also caused 6-7 mV hyperpolarization that is most likely due to a concentrating of intracellular K+. 4. The inhibition of IK helps explain the lengthening of action potentials observed in osmotically stressed heart cells. These results, together with the reported IK stimulation by hyposmotic cell swelling, provide further support for cell volume-sensitive properties of cardiac electrical activity.

Sarcolemmal hydraulic conductivity of guinea-pig and rat ventricular myocytes.

OBJECTIVE: Osmotic gradient-induced volume change and sarcolemmal water permeability of cardiac myocytes were evaluated to characterize the mechanism of water flux across the plasma membranes. METHODS: Cell surface dimensions were measured from isolated guinea-pig and rat ventricular myocytes by digital videomicroscopy, and membrane hydraulic conductivity (L(p)) was obtained by analyzing the time course of cell swelling and shrinkage in response to osmotic gradients. RESULTS: Superfusion with anisosmotic solution (0.5-4 times normal osmolality) caused a rapid (<3 min to steady states) and reversible myocyte swelling or shrinkage. L(p) was approximately 1.9 x 10(-10) l N(-1) s(-1) for guinea-pig myocytes and approximately 1.7 x 10(-10) l N(-1) s(-1) for rat myocytes at 35 degrees C. Arrhenius activation energy (E(a)), a measure of the energy barrier to water flux, was approximately 3.7 (guinea-pig) and approximately 3.6 kcal mol(-1) (rat) between 11 and 35 degrees C; these values are equivalent to E(a) of self-diffusion of water in bulk solution ( approximately 4 kcal mol(-1)). Treatment with 0.1 mM Hg(2+), a sulfhydryl-oxidizing reagent that blocks membrane water channels, reduced L(p) by approximately 80%, and the sulfhydryl-reducing reagent dithiothreitol (10 mM) antagonized the inhibitory action of Hg(2+). Inhibition of the volume-sensitive cation (30 microM Gd3+) and anion (1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonate) channels and Na+-K+ pump (10 microM ouabain) modified the size of osmotic swelling but had little effect on L(p). CONCLUSIONS: Although the observed L(p) is relatively small in magnitude, the low E(a) and the sulfhydryl reagent-induced modification of L(p) are characteristic of channel-mediated water transport. These data suggest that water flux across the sarcolemma of guinea-pig and rat heart cells occurs through parallel pathways, i.e., the majority passing through water channels and the remainder penetrating the lipid bilayers.

Osmometric and water-transporting properties of guinea pig cardiac myocytes.

To elucidate the mechanism of water flux across heart cell membranes, osmotically induced volume changes and sarcolemmal water permeability were evaluated in isolated guinea pig ventricular myocytes by videomicroscopic measurements of cell surface dimensions. Superfusion with anisosmotic solution (0.5-4 times normal osmolality) caused a rapid (lt;3 min to new steady state) and reversible cell swelling or shrinkage mainly because of proportional changes in cell width and thickness. The van't Hoff relationship between relative cell volume and the reciprocal of relative osmolality was linear and predicted an apparent osmotically dead space of approximately 35% cell volume. The osmotic water permeability coefficient (P(f)) measured from the time course of cell swelling/shrinkage was approximately 22 microm.s(-1) at 35 degrees C. Arrhenius activation energy (E(a)), a measure of the energy barrier to water flux, was approximately 3.8 kcal.mol(-1) between 11 and 35 degrees C; this value is equivalent to E(a) for free-water diffusion in bulk solution ( approximately 4 kcal.mol(-1)). Treatment with 0.1 mM Hg(2+), a sulfhydryl-oxidizing reagent, reduced P(f) by approximately 90%, and the sulfhydryl-reducing reagent dithiothreitol (10 mM) antagonized the inhibitory action of Hg(2+). E(a) measured from Hg(2+)-treated myocytes (12.3 kcal.mol(-1)) was in the range of that for diffusional water movement through the lipid bilayers (>10 kcal.mol(-1)). Although the observed P(f) is small in magnitude, both the low E(a) and the sulfhydryl-related modifications of P(f) are characteristic of channel-mediated water transport. These data suggest that water channels form a major conduit for water crossing the sarcolemma of guinea-pig heart cells.