Ouabain-sensitive ion fluxes in the smooth muscle of the guinea-pig's taenia coli

86Rb is not a reliable tracer for potassium in skeletal muscle

For technical reasons, 86Rb is frequently preferred to 42K as a tracer for K+. Systematic comparisons of the two isotopes, however, are rarely done. In this paper we compare the transport of 42K and 86Rb in rat and mouse soleus muscle and in rat erythrocytes. Ouabain-suppressible K+ uptake in rat soleus was the same whether measured with 42K or 86Rb, both when stimulated by insulin, salbutamol and calcitonin-gene-related peptide (CGRP), and when inhibited by graded concentrations of ouabain. Control experiments with rat erythrocytes, where Na(+)-K(+)-Cl- co-transport has earlier been demonstrated, showed closely similar inhibitory effects of bumetanide on 42K and 86Rb uptake. In contrast, bumetanide produced no significant change in 42K uptake of rat and mouse soleus muscle, but clearly inhibited 86Rb uptake at concentrations down to 10(-7) M (P < 0.001). Whereas the addition of 150 mM NaCl had no effect on 42K uptake in rat soleus, 86Rb uptake, and in particular the bumetanide-suppressible component, was markedly increased by this addition. The inhibitory effect of bumetanide on 86Rb uptake gives rise to the false impression that skeletal muscle contains a NaKCl2 co-transport system. Efflux studies showed that the fractional loss of 42K from rat soleus muscle is 2.3 times larger than that of 86Rb. Salbutamol and CGRP increased 86Rb efflux, but inhibited 42K efflux. This implies that for studies of K+ efflux and bumetanide-sensitive K+ transport, 86Rb is not even an acceptable tracer for the detection of qualitative changes. Control experiments with 42K are essential in any characterization of unknown K+ transport processes.

The effect of cromakalim on the smooth muscle of the guinea-pig urinary bladder

1. The actions of cromakalim were studied on the detrusor muscle from guinea-pig urinary bladder. Cromakalim reduced the frequency and amplitude of spontaneous contractile activity of the smooth muscle of the guinea-pig urinary bladder at 5 x 10(-8)M and abolished the activity at concentrations above 5 x 10(-7)M. 2. Electrophysiological experiments demonstrated that cromakalim increased membrane conductance, caused a dose-dependent hyperpolarization of the cell membrane and loss of spike activity. These events are consistent with the opening of K+ channels. 3. The effects of 10(-6)M and 10(-5)M cromakalim on the contractile responses to carbachol, potassium and transmural nerve stimulation were studied. Cromakalim did not prevent the detrusor from responding to these agents, although it significantly reduced the contractile response to K+ at concentrations below 70 mM. 4. Uptake and efflux experiments using 86Rb+ were unable to demonstrate any significant effect on transmembrane movement produced by cromakalim (10(-5)M). 5. 43K+ efflux showed a dose-dependent increase in the rate constant on addition of cromakalim. The difference in the selectivity for K+ over Rb+ was confirmed in dual label uptake experiments. 6. Substitution experiments in which the K+ ions in the tissue were gradually replaced by Rb+ demonstrated that cromakalim had a progressively decreasing effect on spontaneous activity as internal K+ was lowered. When all the K+ was replaced by Rb+, cromakalim no longer inhibited spontaneous activity, confirming that the channel opened by cromakalim appears relatively impermeant to Rb+.

Altered biochemical and functional responses in aorta from hypertensive rats

Factors that lead to supersensitivity of vascular smooth muscle to norepinephrine during aldosterone-salt-induced hypertension in rats appear to reside beyond ligand-alpha-adrenergic receptor binding, which we have shown previously to be normal. The objective of this study was to determine whether significant shifts occur in the coupling between receptors and the production of putative second messengers. Measures of [3H]myo-inositol phosphates in aorta (endothelium removed) exhibited a concentration-dependent increase to norepinephrine, with the 50% response shifted significantly to the left in the hypertensive group (7.0 +/- 0.9 X 10(-7) M in 8 control rats vs 1.1 +/- 0.2 X 10(-7) M in 8 hypertensive rats; p less than 0.001). The production of [32P]phosphatidic acid was also shifted (6.5 +/- 2.5 X 10(-7) M in 16 control vs 1.9 +/- 0.8 X 10(-7) M in 12 hypertensive rats; p less than 0.05). The functional responses of 42K efflux and contraction to norepinephrine were also significantly shifted threefold to 15-fold in the hypertensive group (p less than 0.001), but the 50% response typically occurred at a 10 to 100 times lower concentration than that for the production of myo-inositol phosphates and phosphatidic acid. The amplification between receptor occupancy and functional responses apparently occurs beyond the production of phosphoinositide metabolites. The fivefold shift in the 50% response of biochemical end points for the hypertensive group accounted for most of the shift (sixfold) in the functional end points.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium release by noradrenaline from central sarcoplasmic reticulum in rabbit main pulmonary artery smooth muscle

The subcellular composition of relaxed and noradrenaline-contracted rabbit main pulmonary artery smooth muscle cells was measured by electron probe X-ray microanalysis of cryosections of rapidly frozen tissue. Some of the preparations were made permeable with saponin and exposed to a known free Ca ion concentration, rapidly frozen, freeze-substituted, and also analysed by electron probe X-ray microanalysis. 98% of intracellular K could be replaced by Rb. This was done to remove the K peak that partially overlaps the Ca peak in the X-ray spectra. The final [Rb]i plus residual [K]i was not significantly different from the [K]i of normal tissue. The [Ca]i in Rb-containing tissue was not significantly different from the [Ca]i in normal, K-containing tissue. Non-mitochondrial micro-regions containing high [Ca] (up to 33 mmol/kg dry wt.) were found at sites 200 nm or more away from the plasma membrane. These micro-regions also contained high [P]. We consider the identification of these regions containing high [Ca] as sarcoplasmic reticulum (s.r.), validated by: (a) conventional electron micrographs that show no other structures in main pulmonary artery smooth muscle in sufficient quantity and location to account for the frequency of these regions, (b) the previous localization of strontium, a functional calcium analogue, in the central s.r. in these smooth muscles (Somlyo & Somlyo, 1971 a), (c) the present demonstration that the central s.r. in this tissue can accumulate large amounts of calcium oxalate. The proportion of regions containing high [Ca] (greater than 12.0 mmol/kg dry wt.) was significantly higher in relaxed (35 of 330 measurements) than in the contracted (14 of 337) tissues (P less than 0.005), or 26 of 34 vs. 6 of 31 high [Ca] measurements in regions identified as s.r. through their high phosphorus content (P less than 0.006). This difference is thought to represent Ca release from the central s.r. There was no significant difference (P greater than 0.05) between the distributions of P in relaxed and contracted smooth muscle. The total cell [Ca]i in relaxed Rb-containing tissue, measured with randomly positioned small probes (3.6 mmol/kg dry wt.), was the same as that measured with large defocused probes, indicating the validity of random sampling. A mathematical model was used to estimate the frequency of including s.r. (35 nm diameter and 5% of cell volume) by a randomly positioned electron probe (50 nm), because we could not visualize s.r. in the cryosections.(ABSTRACT TRUNCATED AT 400 WORDS)

Thallium uptake by canine iliac arteries

Most of the thallium uptake by canine iliac arteries (70%) was inhibited by ouabain. The component sensitive to ouabain was significantly increased by incubating tissues in K+-free solution and reduced by low temperatures, metabolic inhibitors, and loading with lithium. The apparent Km for thallium for the ouabain-sensitive uptake was 0.47 +/- 0.046 mM. External K+ inhibited thallium uptake and the apparent Ki was estimated to be 6.5 mM. Thallium thus appears to have a greater affinity for the ouabain-sensitive components. The residual component of thallium uptake was unaffected by maneuvers that affected the ouabain-sensitive uptake, which thus appears to be a suitable indicator of Na+ pump activity in vascular smooth muscle.

The effects of rubidium ions and membrane potentials on the intracellular sodium activity of sheep Purkinje fibres

1. Intracellular Na activity, aiNa, was measured in voltage-clamped sheep cardiac Purkinke fibres. 2. Increasing [Rb]0 from 0 to 4 mM in K-free solutions (at a fixed membrane potential) decreased aiNa. Further increases of [Rb]0 (up to 20 mM) had little or no effect. 3. Following exposure to Rb-free, K-free solution, the addition of a test concentration of Rb produced an exponential decrease of aiNa. The rate constant of decay of aiNa increased with increasing [Rb]0 over the measured range (0-20 mM). 4. The accompanying electrogenic Na pump current transient decayed with the same rate constant as aiNa over the range of [Rb]0 examined. During this decay the electrogenic Na pump current was a linear function of aiNa. Increasing [Rb]0 increased the steepness of the dependence of the electrogenic current on aiNa. 5. A constant fraction of the net Na efflux was electrogenic. This fraction was not affected by varying [Rb]0 over the range 0-20 mM. 6. Using a simple model, it is shown that the dependence of steady-state aiNa on [Rb]0 is half-saturated by less than 1 mM-[Rb]0. The rate constant of decay of aiNa and the slope of the relationship between electrogenic Na pump current and aiNa are, however, better fitted with a lower affinity for Rb (K0.5 = 4 mM-[Rb]0). 7. Depolarizing the membrane potential with the voltage clamp decreased aiNa; hyperpolarization increased it. These effects persisted in the presence of 10(-5) M-strophanthidin. An effect of membrane potential on the net passive Na influx can account for the observations. 8. The effects of membrane potential on the net passive Na influx were examined by measuring the maximum rate of rise of aiNa at different holding potentials after inhibiting the Na-K pump in a K-free, Rb-free solution. Depolarization decreased the Na influx. 9. Using the constant field equation, the net passive Na influx was used to estimate the apparent Na permeability coefficient, PNa. This was between 0.8 x 10(-8) and 1.5 x 10(-8) cm sec-1.

The kinetics of ouabain-sensitive ionic transport in the rabbit carotid artery

1. Ouabain (0.1 mM)-sensitive 42K influx and 24Na efflux have been measured in rabbit carotid arteries under conditions of high cellular potassium, [K]i, as well as sodium, [Na]i. About 50% of the total fluxes are ouabain-sensitive (active) under conditions of high [K]i. 2. The extracellular space, determined by 60Co-EDTA, was relatively large in comparison to cellular water. The ionic concentrations in normal solution, estimated from isotope flux components, are: [Na]i = 24; [K]i = 169; [Cl]i = 68 mmol/l cell water. 3. The ouabain=sensitive 42K influx and 24Na efflux in high-K tissues were measured at varying external concentrations of potassium, [K]o, and normal concentrations of external sodium, [Na]o. Sigmoidal kinetics were observed and fitted to a co-operative interaction model. The maximal efflux of 24Na, 0.245 muequiv/g wet weight per minute, was about 1.4 times that for 42K influx. Half-maximal stimulation was achieved at [K]0.5o of 2.4 mM for Na, and 3.4 mM for K transport. The flux ratio of Na to K approximated 1.5. 4. Increased 42K efflux was found in the presence of ouabain and the passive influx of 42K was corrected for this effect. In the absence of this correction the ouabain-sensitive 42K influx would be reduced, and the Na/K flux ratio raised to about 2. 5. The [K]o-dependence of ouabain-sensitive fluxes was measured on Na-loaded tissues. 24Na efflux exhibited saturation kinetics with a maximum of 1.18 muequiv/g wet weight per minute and [K]0.5o = 3.1 mM. The 42K influx was two thirds the active Na efflux for [K]o less than or equal to 5 mM. At high [K]o, however, the influx greatly exceeded the predicted levels. Evidence is presented for a ouabain-sensitive membrane hyperpolarization being responsible for an additional influx of 42K. 6. The ouabain-sensitive 24Na efflux showed a sigmoidal dependence on [Na]i in the presence of [K]o = 10 mM and normal [Na]o. The maximal efflux was 0.88 muequiv/g weight per minute and [Na]0.5i = 49 mmol/l cell water, which is about twice the physiological operating point. 7. It is concluded that active Na and K transport in rabbit carotid artery follow sigmoidal kinetics and the flux ratio is about 1.5. Changes in [K]o and [Na]i over the physiological range can markedly affect transport, and may regulate vascular contraction by their action on electrogenic transport.

Ouabain-sensitive thallium fluxes in smooth muscle of rabbit uterus

1. Rabbit myometrium accumulates Tl in a time-dependent fashion and the majority of the uptake of Tl is ouabain-sensitive. 2. In normal chloride-containing media, the uptake of Tl, though ouabain-sensitive, is less than in chloride-free media and this difference is due to a greater ouabain-insensitive uptake. 3. The ouabain-insensitive uptake in normal chloride-containing media is reduced by furosemide and furosemide also reduces total uptake in this solution. In chloride-free media, however, furosemide is without effect on total or ouabain-sensitive uptake. 4. In chloride-free media, the uptake of Tl against Tl concentration is sigmoidal, suggesting that more than one Tl ion is being transported at a time. 5. Tl was capable of substituting for K in electrogenic Na pumping; it was approximately twice as effective as K and the inhibitory effect of Tl was blocked by ouabain. 6. Tl efflux can be explained by a simple two-compartment model in both normal and chloride-free solutions. 7. The uptake of Tl was inhibited by alkali cations with the order of potency being Tl+ > K+ > Rb+ > Cs+ at 10 mM and Tl+ > K4 = Rb+ > Cs+ at 5 mM ion concentrations. 8. It is concluded that Tl enters the smooth muscle of rabbit uterus by diffusion, active ouabain-sensitive transport and active chloride- and furosemide-sensitive transport.

Replacement of K+ with Rb+ or Cs+, and its effects on the mechanical responses to norepinephrine and methacholine in the rat vas deferens

The rat vas deferens was stored overnight in cold, K+-free Krebs solution to deplete intracellular K+ then incubated in K+-, Rb+-, or Cs+-containing Krebs solution at 37 degrees C to load these ions inside the cells. After 4 h, the contents of K+ or Rb+ reached the level of K+ in the fresh vas deferens; the content of Cs+ was less than half that of the fresh vas deferens. Dose-response curves to norepinephrine and methacholine were determined under these conditions, and the curves in Rb+ or Cs+ solution were compared with those in K+ solution. The cold storage per se had little effect on the dose-response curves in K+ solution except that it slightly decreased the maximal response to norepinephrine. The dose-response curves in Rb+ solution were to the left of those in K+ solution. The maximal response to methacholine was greatly increased. On the other hand, the dose-response curves in Cs+ solution were to the right of those in K+ solution. The maximal responses were greatly decreased with both drugs. The results suggest that Rb+ but not Cs+ can fully substitute for K+ in the rat vas deferens response to norepinephrine and methacholine.

Pharmacology of the adrenoceptors and cholinoceptors of the BC3H1 nonfusing muscle cell line

1. We have studied the action of different transmitters on the transmembrane 42K or 86Rb efflux from tissue cultures of the BC3H1 muscle cell line. 2. The effect of catecholamines and carbachol (CCh) on the isotope efflux rate was measured by addition of the drugs at different times during the washout. 3. Noradrenaline (NA), phenylephrine (Phe), isoprenaline (Iso) and CCh increased 42K and 86Rb efflux rate in a dose-dependent manner. 4. The action of NA seems to be due exclusively to the stimulation of alpha-receptors, since its effect was blocked by phentolamine but not by propranolol. The effects of Iso on the 86Rb efflux were inhibited by propranolol. The beta-receptors in the BC3H1 cells seem to be the beta2-type since they are stimulated by Iso and insensitive to NA. 5. The effect of CCh was blocked (+)-tubocurarine but not by atropine. This result confirms the presence of nicotinic receptors in BC3H1 cells.