Electrical and mechanical responses to inhibition of cell respiration in vascular smooth muscle of the rat portal vein

Effect of hypoxia on force, intracellular pH and Ca2+ concentration in rat cerebral and mesenteric small arteries

1. The effect of severe hypoxia on force, intracellular Ca2+ concentration ([Ca2+]i) and pHi was studied in isolated small arteries from rat brain and rat mesenterium. The arteries were mounted for isometric force recording while [Ca2+]i was measured with fura-2 or pHi was measured with bis-carboxyethylcarboxyfluorescein (BCECF). 2. Hypoxia reduced the force development in response to arginine vasopressin (AVP) while [Ca2+]i was unchanged or only slightly reduced. Inhibition of acid extrusion by omission of sodium caused no force development in mesenteric arteries, but the fall in pHi was enhanced during hypoxia. In cerebral arteries, hypoxia reduced the force development associated with omission of sodium, and the fall in pHi was less than during normoxic conditions. When acid extrusion was intact, pHi was not affected by hypoxia and the changes in pHi during activation with AVP were similar during hypoxia and in the control situation. 3. Although a decrease in smooth muscle [Ca2+]i may be partly responsible for the reduced force development during hypoxia, [Ca2+]i-independent mechanism(s) may play an even more important role. Furthermore, although hypoxia and force development are associated with enhanced acid production, acid extrusion maintains pHi near the control level and it is unlikely that a decrease in smooth muscle pHi plays any role in the reduced force development during hypoxia.

K channel activation by nucleotide diphosphates and its inhibition by glibenclamide in vascular smooth muscle cells

1. Whole-cell and inside-out patch recordings were made from single smooth muscle cells that had been isolated enzymatically and mechanically from the rabbit portal vein. 2. In whole-cells the inclusion in the recording pipette solution of nucleotide diphosphates (NDPs), but not tri- or monophosphates, induced a K-current that developed gradually over 5 to 15 min. Intracellular 1 mM guanosine 5'-diphosphate (GDP) induced a slowly developing outward K-current at -37 mV that reached a maximum on average of 72 +/- 4 pA (n = 40). Half maximal effect was estimated to occur with about 0.2 mM GDP. Except for ADP, other NDPs had comparable effects. At 0.1 mM, ADP was equivalent to GDP but at higher concentration ADP was less effective. ADP induced its maximum effect at 1 mM but had almost no effect at 10 mM. 3. In 14% of inside-out patches exposed to 1 mM GDP at the intracellular surface, characteristic K channel activity was observed which showed long (> 1 s) bursts of openings separated by longer closed periods. The current-voltage relationship for the channel was linear in a 60 mM:130 mM K-gradient and the unitary conductance was 24 pS. 4. Glibenclamide applied via the extracellular solution was found to be a potent inhibitor of GDP-induced K-current (IK(GDP)) in the whole-cell. The Kd was 25 nM and the inhibition was fully reversible on wash-out. 5. IK(GDP) was not evoked if Mg ions were absent from the pipette solution. In contrast the omission of extracellular Mg ions had no effect on outward or inward IK(GDP). 6. Inclusion of 1 mM ATP in the recording pipette solution reduced IK(GDP) and also attenuated its decline during long (25 min) recordings. 7. When perforated-patch whole-cell recording was used, metabolic poisoning with cyanide and 2-deoxy-D-glucose induced a glibenclamide-sensitive K-current. This current was not observed when conventional whole-cell recording was used. Possible reasons for this difference are discussed. 8. These K channels appear similar to ATP-sensitive K channels but we refer to them as nucleotide diphosphate-dependent K channels (KNDP) to emphasise what seems to be a primary role for nucleotide diphosphates in their regulation.

Effects of metabolic inhibition on cytoplasmic calcium and contraction in smooth muscle of rat portal vein

Contractions in the rat portal vein, evoked by spontaneous action potentials or depolarizing high-K+ solution, are rapidly and reversibly inhibited by hypoxia or respiratory blockade. Intracellular free calcium ([Ca2+]i) was measured using Fura-2 to evaluate the effects of metabolic blockade on excitation-contraction coupling. Spontaneous contractions were associated with transient increases in [Ca2+]i. During exposure to cyanide (0.2-0.4 mM) or 2,4-dinitrophenol (30 microM) the duration and amplitude of the Ca2+ transients were decreased, leading to a decreased mean time integral of the individual [Ca2+]i transient, and corresponding decrease in the duration and amplitude of the contraction. Basal [Ca2+]i was increased in the presence of the metabolic inhibitors. High-K+ (40 mM) contractions caused a sustained increase in [Ca2+]i, which was not inhibited by exposure to cyanide, although the amplitude of the associated contraction was greatly reduced. Together with the earlier demonstration of decreased 20 kD myosin light chain phosphorylation under these conditions, this indicates that the activation of contraction is influenced by metabolism via the energy dependence of the light chain phosphorylation reaction. Thus at least three steps in the excitation-contraction sequence are influenced by inhibition of oxidative metabolism: membrane excitation, light chain phosphorylation, and the cross-bridge cycle. This provides mechanisms for a high degree of metabolic sensitivity of vascular tone, of importance for the adaptation of blood flow to tissue metabolic demands.

Metabolic correlates to pacemaker activity in the smooth muscle of guinea-pig mesotubarium

Oxygen consumption (FO2) and lactate production (Flac) were measured during spontaneous activity in the guinea-pig mesotubarium. During spontaneous contractions FO2 increased to maximally 0.270 +/- 0.025 mumol min-1g-1 (n = 23), followed by a rapid fall immediately upon relaxation. In the relaxed period (5-15 min) between spontaneous contractions FO2 continued to slowly decrease by about 25% towards a final value of 0.150 +/- 0.01 mumol min-1g-1. Flac showed no consistent variation during the relaxed period. Ouabain (10(-6)M) produced a contracture, which was abolished by the Ca2(+)-antagonist felodipine (10(-6)M). In the presence of felodipine, addition of ouabain caused depolarization and a decrease of oxygen consumption by 21% and of lactate production by 31%. Exchange of glucose in the physiological Krebs solution for beta-hydroxybutyrate did not influence spontaneous activity, while subsequent addition of cyanide (2 mM) abolished contractions and caused a hyperpolarization of 15 mV. Blockade of ATP-dependent K+ channels by addition of glibenclamide (10(-7)M) to the relaxed muscle in this situation caused spontaneous contractile activity to reappear. In glucose-containing Krebs solution glibenclamide had no effect on the spontaneous contractile and electrical activity and contractions persisted after addition of cyanide. The relaxing and hyperpolarizing effect of pinacidil could be counteracted by addition of glibenclamide. The results suggest that a decrease in electrogenic Na+/K(+)-pump activity in the course of the relaxed period between contractions could contribute to the pacemaker behaviour. ATP-dependent K+ channels, while having little influence on the spontaneous contractile activity under normal metabolic conditions, could be activated during blockade of aerobic and anaerobic metabolism, leading to inhibition of pacemaker activity.