Beta-adrenergic effects on rat myometrium: mechanisms of membrane hyperpolarization

Diversity of inhibitory responses to beta2-stimulants shown by term-pregnant human myometria in vitro is partly due to differences in receptor density

OBJECTIVE

The aims of this study were (1) to evaluate the usefulness of the new beta2-adrenergic stimulant KUR-1246 as a tocolytic agent and (2) to clarify the mechanisms that underlie the diverse inhibitory effects of beta2-stimulants that are seen in human myometria in vitro.

STUDY DESIGN

The displacement of tritiated ([3H]) (-)CGP 12177 (0.4 nmol/L) by KUR-1246 and other beta2-stimulants was examined with human beta(1)- and beta2-receptors present on membrane fractions. The inhibitory effects of these beta2-stimulants on the term-pregnant human myometrium were compared with the use of isometric tension recording and microelectrode methods. Finally, the relationship between [3H]dihydroaloprenolol binding and the magnitude of the tocolytic effect of isoproterenol was examined.

RESULTS

KUR-1246 was approximately 80 times and 7 times more selective for beta2-receptors than isoproterenol and ritodrine, respectively. The inhibitory effect of KUR-1246 was as strong as the inhibitory effect of the conventional beta2-adrenergic stimulants. A wide range of inhibitory effects was observed, even when high concentrations of isoproterenol or KUR-1246 were applied. There was a correlation between the degree to which isoproterenol suppressed contractions and the number of [3H]dihydroaloprenolol binding sites on the membrane in each muscle strip.

CONCLUSION

KUR-1246 should be a very useful beta2-adrenergic stimulant for use as a tocolytic agent because of its high selectivity for the beta2-receptor and its potent inhibitory effect. The diversity of the inhibitory effects that are induced by beta2-stimulants is at least partly due to differences in beta2-receptor density among term-pregnant human myometria.

Differential toxic effects of gentamicin on cultured renal epithelial cells (LLC-PK1) on application to the brush border membrane or the basolateral membrane

Aminoglycoside antibiotics are generally accepted to accumulate in renal proximal tubule cells from the luminal surface and show toxic effects on the cells. The binding affinity and membrane permeability of aminoglycoside antibiotics are different at the brush border membrane (BBM) and the basolateral membrane (BLM) of proximal tubule cells. This study was performed, therefore, to investigate the differential effects of the aminoglycoside antibiotic gentamicin (GM) on cultured LLC-PK1 cells, a pig kidney proximal epithelial cell line, after addition to the BBM or the BLM side. LLC-PK1 cells were cultured on microporous membranes until forming confluent monolayers, and then GM was added to either the BBM or the BLM side. GM caused release of enzymes from the organelles, with a higher level of release observed following addition to the BBM side than that to the BLM side. Patterns of [3H]GM uptake by the cells differed in a manner dependent on whether it was added to the BBM or the BLM side. That is, the cellular uptake from the BBM side increased with incubation time, while that from the BLM side showed rapid saturation. These results suggested that aminoglycoside antibiotics show differential effects on cultured proximal epithelial cells and have differential patterns of cellular uptake when added to the BBM or the BLM side.

Enhancement of K+ currents by stimulation of protein kinase C in the uterine smooth muscle cells of the pregnant rat

1. Effects of phorbol esters on the K+ currents in isolated rat uterine smooth muscle cells during (18-day) pregnancy were examined using whole-cell voltage-clamp modes. All experiments were performed at room temperature. 2. Test pulses were applied between -20 to + 90 mV from a holding potential of -40 mV. Initially, a transient outward current (ITO) was activated, and outward K+ current (IK) was followed. Threshold potential was - 10 to 0 mV, and the activation was voltage-dependent. At - 80 mV, ITO and IK were 17.8 +/- 3.3 and 13.2 +/- 2.6 pA/pF as a current density. Membrane capacitance was 64.0 +/- 11.5 pF (n = 8). 3. At 0.1 microM, 12-O-tetradecanoylphorbol-13-acetate (TPA) and 4-beta-phorbol-12-13-dibutyrate (PDB) enhanced IK at +80mV by 14.5 +/- 2.0% (n = 8, P < 0.05) and 23.5 +/- 2.2% (n = 7, P < 0.01). Also, ITO at +80mV was increased by 22.1 +/- 2.1% (n = 8, P < 0.01) at 1 microM TPA and by 22.7 +/- 3.0 (n = 7, P < 0.05) at 0.1 microM PDB, significantly. 4. These results indicate that the IK and ITO currents are present in the uterine smooth muscle cells of pregnant rat, and PK-C stimulation modulates the K+ currents, resulting in the regulation of physiological contraction of the uterine muscle during pregnancy.

The lack of a role for potassium channel opening in the action of relaxin in the rat isolated uterus; a comparison with levcromakalim and salbutamol

1. The effects of relaxin in vitro in the isolated uterus from the non-pregnant rat were compared with those of levcromakalim and salbutamol in tissue bath, 42K+ -efflux and electrophysiological studies, to determine whether relaxin exhibits the characteristics of an opener of KATP-channels. 2. In uterus exposed to oxytocin (0.2 nM), tetraethylammonium (TEA, 10 mM) and glibenclamide (10 microM) produced large rightward shifts of the log10 concentration-effect curve to levcromakalim (125 fold and 118 fold, respectively). TEA (10 mM) caused only small rightward shifts of the log10 concentration-effect curves to salbutamol and relaxin (5.2 fold and 7.5 fold respectively). Glibenclamide did not antagonize salbutamol or relaxin. 3. Levromakalim (0.2 and 2 microM) suppressed the spasm evoked by low ( < or = 40 mM) but not high ( > 40 mM) concentrations of KCl. Salbutamol (1.5 nM) inhibited the spasm evoked by low concentrations of KCl ( < or = 40 mM). Salbutamol (15 nM) and relaxin (3 and 30 nM) inhibited the spasm evoked by low and high concentrations of KCl (10-80 mM). 4. Relaxin (0.12 microM) did not produce an increase in 42K+-efflux from longitudinal segments of rat myometrium. Exposure of tissues to relaxin (0.12 microM), in the presence of diltiazem (1 microM) plus KCl (20 mM), resulted in a small increase in 42K+-efflux of short duration. 5. Electrophysiological recording showed that the phasic spasms of the uterus exposed to oxytocin (0.2 nM) were accompanied by bursts of spiking activity superimposed upon a plateau potential. Inhibition of the mechanical activity of the uterus by levcromakalim (2 and 10 microM), salbutamol (30 nM) or relaxin (0.18 microM) was accompanied by a reduction in the duration of the plateau potential and the number of spikes without membrane hyperpolarization. 6. Unlike levcromakalim, relaxin did not selectively inhibit the spasm evoked by low concentrations of KCl and was not markedly antagonized by TEA or glibenclamide. Under conditions where a cromakalim-induced increase of the 42K+-efflux rate has been demonstrated, relaxin had only a very small effect. In isolated uterus from the rat, in contrast to observations in vivo, relaxin did not exhibit the characteristics of an opener of KATP-channels suggesting that another mechanism accounts for its inhibitory action.

The involvement of ATP-sensitive potassium channels in beta-adrenoceptor-mediated vasorelaxation in the rat isolated mesenteric arterial bed

1. We have used the isolated buffer-perfused superior mesenteric arterial bed of the rat to assess the involvement of ATP-sensitive potassium (KATP) channels in the vasorelaxant responses to beta-adrenoceptor agonists. 2. The vasorelaxant potencies of the non-selective beta-adrenoceptor agonist, isoprenaline, the beta 1-adrenoceptor agonist, dobutamine and the beta 2-adrenoceptor agonist, terbutaline were all significantly (P < 0.05) reduced (isoprenaline, ED50 = 265 +/- 31 pmol v. 1.05 +/- 0.42 nmol; dobutamine, ED50 = 294 +/- 67 pmol v. 497 +/- 115 pmol; terbutaline, ED50 = 157 +/- 26 nmol v. 452 +/- 120 nmol) in the presence of the KATP-channel blocker, glibenclamide. 3. The presence of glibenclamide only weakly influenced the vasorelaxant properties of salbutamol, a beta 2-adrenoceptor agonist, while those of verapamil, a beta-adrenoceptor-independent vasorelaxant, were unaffected. 4. In radioligand binding experiments, glibenclamide (1 nM-100 microM) did not displace any specific [3H]-dihydroalprenolol binding from rat beta-adrenoceptors. Therefore, glibenclamide does not bind to beta-adrenoceptors at the concentration used in the present investigation. 5. Vasorelaxant responses to dibutyryl cyclic AMP, the cell permeable analogue of cyclic AMP, were also unaffected by glibenclamide, indicating that the coupling of beta-adrenoceptors to KATP-channels occurs independently of the elevation of intracellular cyclic AMP. 6. We have shown that a significant element of the vasorelaxant responses to both beta 1- and beta 2-adrenoceptor activation involves the opening of KATP-channels. In conclusion, KATP-channels may play a physiological role in beta-adrenoceptor-mediated vasodilatation.

Ion channels and the control of myometrial electrical activity

Understanding the role of ion channels in the generation of slow waves and action potentials in the myometrium is critical in designing strategies to regulate uterine contractile activity. The development of the patch clamp technique has allowed the identification of specific types of channels in the myometrium and provided insights into their regulation by hormones and drugs. Specifically, new studies suggest that KATP and KCa channel openers could be important tools in the management of inappropriate uterine contractions, but peripheral effects will have to be controlled. Conversely, blockers of these same channels may have some effects on dystocia. The study of contractant-operated channels in the myometrium is still in its infancy, but promises new insights into possible modes of regulation as well. Myometrial activity is controlled at a number of levels. The regulation of ion channels is an important aspect, but receptor-mediated actions that do not appear to be voltage- or ion-dependent presumably are also important contributors and hence are sites of potential modulation as well. Clearly, future multifaceted approaches to tocolysis, and perhaps also dystocia, may well include agents targeting the activity of ion channels.

Activation of K+ channels by ritodrine hydrochloride in uterine smooth muscle cells from pregnant women

This study investigated the mechanism of activation of K+ channels by ritodrine hydrochloride in human myometrial smooth muscle cells. The patch-clamp technique was used for recording single channel currents. Ritodrine (10(-5) M) activated two types of K+ channels in cultured uterine smooth muscle cells from pregnant women: the Ca(2+)-activated K+ (KCa) channel and the ATP-sensitive K+ (KATP) channel. Forskolin (10(-4) M), an activator of adenylate cyclase, and protein kinase A activated the KCa channel. In addition, 10(-4) M GTP activated the KCa channel in inside-out patches using a pipette containing 10(-5) M ritodrine. The KATP channel was activated by protein kinase A, but not by 10(-4) M GTP. The beta-adrenoceptor agonist ritodrine activates two types of K+ channels: the KCa channel via direct gating by GTP-binding proteins and possibly via cAMP-dependent phosphorylation, and the KATP channel possibly via cAMP-dependent phosphorylation. These mechanisms partially explain the relaxing effect of ritodrine hydrochloride.

Forskolin inhibition of K+ current in pregnant rat uterine smooth muscle cells

Two kinds of outward K+ currents were examined in single smooth muscle cells from pregnant rat uterus, using whole-cell voltage clamp. The first and faster component was more sensitive to 4-aminopyridine (4-AP), whereas the second and slower (delayed) component was more sensitive to tetraethylammonium (TEA). A possible third K+ component (Ca activated K+ current) was not recorded as the pipette solution included EGTA. Forskolin inhibited the outward current in a concentration-dependent manner (50% inhibition occurred at about 30 microM); it affected the delayed component rather than the fast component. 8-Bromo-cAMP did not alter the outward current. In addition, inhibitors of protein kinase A and GDP-beta S and GTP-gamma S did not affect the forskolin-induced inhibition. These results indicate that forskolin inhibition of the delayed component of the outward current is independent of cAMP generation in the pregnant rat myometrial cells. Therefore, forskolin seems to directly inhibit specific K+ channels, as was reported for several other cell types.

Ca(2+)-activated K+ channels in pregnant rat myometrium: modulation by a beta-adrenergic agent

The properties of Ca(2+)-activated K+ currents and channels were characterized in pregnant rat myometrium in whole cell and cell-attached patches and in lipid bilayers. Membrane depolarization of cultured myometrial cells from a holding potential of -50 to +70 mV in 10-mV steps under voltage-clamp conditions (whole cell mode) activated K+ outward currents (IK). At +70 mV, in the presence of 0.2 mM external Ca2+, the amplitude and activation time constant of IK were 15.0 +/- 2.1 microA/microF and 1.5 +/- 0.2 ms, respectively. Addition of 1 microM A23187 to the external solution increased the current from a control value of 16.0 +/- 2.0 to 67.9 +/- 9.1 microA/microF. Charybdotoxin, a blocker of Ca(2+)-activated K (KCa) channels, and a low concentration of tetraethylammonium chloride (TEA; 1 mM) decreased the amplitude of IK by 47 and 62%, respectively. In cell-attached patches from these cells, 1 microM A23187 increased the open time probability of a 143 +/- 6.0 pS K+ channel. Incorporation of plasma membrane vesicles from pregnant myometrium into lipid bilayers resulted in one predominant type of K+ channel. The unitary conductance of the K+ channel was 326 +/- 9.0 pS in symmetrical 450 mM KCl. The channel activation was both voltage and Ca2+ dependent. TEA inhibited the channel activity with a dissociation constant (Kd) of 378 +/- 10 microM at -60 mV or 1,477 +/- 80 microM at +60 mV. The whole cell currents were found to be stimulated by isoproterenol, a beta-adrenergic agent.(ABSTRACT TRUNCATED AT 250 WORDS)

On the pharmacological and physiological role of glibenclamide-sensitive potassium channels in colonic smooth muscle

Actions of activators of glibenclamide sensitive K+ channels on canine colonic circular muscle were investigated. Cromakalim as well as its (-) enantiomer lemakalim caused inhibition of spontaneous phasic contractile activity (EC50's 4.4 +/- 0.1 x 10(-7) M and 2.3 +/- 0.4 x 10(-7) M, respectively) and of carbachol induced activity (EC50's: 9.4 +/- 5.1 x 10(-7) M and 4.3 +/- 1.4 x 10(-7) M, respectively). Cromakalim and lemakalim effects were completely inhibited by glibenclamide. Additive effects between K+ channel activators and other drugs relaxing colonic muscle (the L-type calcium channel blocker D600 and forskolin) were seen. A physiological role for specific glibenclamide sensitive K+ channels, if existing, remains unresolved. The present study indicates that the non-adrenergic inhibitory nerves do not act through these channels, neither does stimulation of muscarinic or beta-adrenergic receptors.

Beta-adrenergic effects on cellular Na, Mg, Ca, K and Cl in vascular smooth muscle: electron probe analysis of rabbit pulmonary artery

The effects of beta-adrenergic stimulation on the cellular content and subcellular distribution of Na, Mg, Ca, K and Cl were determined by electron probe X-ray microanalysis of muscles stimulated with 5-hydroxytryptamine. Isoproterenol caused a significant decrease in cytoplasmic and mitochondrial Na and Cl, and an increase in cytoplasmic Mg. Isoproterenol also significantly decreased total cytoplasmic Ca measured with small diameter probes, without affecting cellular Ca measured with large probes that included the sarcoplasmic reticulum (SR). The decrease in cytoplasmic Na and the effects on cytoplasmic and cellular Ca are consistent with, respectively, beta-adrenergic stimulation of the Na-pump and of Ca-uptake into the SR, but the beta-adrenergic increase in cytoplasmic Mg also raises the possibility of stimulated Na/Mg exchange.

Influence of terbutaline on ovine uterine response to prostaglandin E2 challenge

OBJECTIVE

Our purpose was to test the effects of terbutaline on uterine electric and contractile responses to prostaglandin E2.

STUDY DESIGN

In five late-gestation ewes, prostaglandin E2 (22.9 +/- 2.3 micrograms/min for 3 minutes) was given twice at 30-minute intervals during control. Terbutaline sulfate (2 micrograms/min) was then infused for 30 minutes. Prostaglandin E2 challenge was repeated 10 minutes after the onset of terbutaline infusion and thereafter at 30-minute intervals. Two-way analysis of variance for repeated measures revealed a significant main effect for time (p less than or equal to 0.0001) and between time and response (p less than or equal to 0.05).

RESULTS

Both electric (p less than or equal to 0.0001) and intrauterine pressure (p less than or equal to 0.0001) responses were suppressed during terbutaline. The influence on intrauterine pressure persisted 10 minutes after terbutaline (p less than or equal to 0.01) while the electric response was not different from control.

CONCLUSIONS

Terbutaline initially diminishes both uterine contractile and electric activity, but electric recovery precedes contractile recovery.

One way cross tolerance between cromakalim and salbutamol in the uterus of the rat in vivo

1. Cross tolerance between the potassium (K+) channel opener, cromakalim and the beta 2-adrenoceptor agonist, salbutamol, was investigated in the uterus of the non-pregnant rat in vivo. Uterine sensitivity to salbutamol was similar in both vehicle-treated and cromakalim-tolerant rats. In salbutamol-tolerant rats, uterine responses to cromakalim were markedly decreased compared with saline-infused rats, such that maximum inhibition of uterine contractions was less than 40%. 2. Propranolol treatment and salbutamol tolerance each produced similar reductions in sensitivity of the uterus to salbutamol of approximately 10 fold. The same dose of propranolol did not influence uterine sensitivity to cromakalim, which suggests that the relaxant action of cromakalim is not due to a direct or indirect activation of uterine beta 2-adrenoceptors. 3. Salbutamol produced a marked (11.7 fold) increase in uterine adenosine 3':5'-cyclic monophosphate (cyclic AMP) concentrations measured ex vivo, which was completely inhibited by propranolol pretreatment, but was unaffected by glibenclamide pretreatment. Cromakalim did not increase uterine cyclic AMP concentrations, suggesting that stimulation of adenylate cyclase is not significant in the uterine relaxant action of cromakalim. 4. The lack of propranolol antagonism of cromakalim and of cromakalim-induced changes in uterine cyclic AMP concentrations suggests that the cross tolerance observed between salbutamol and cromakalim may be at the level of K(+)-channels.

Effects of magnesium and terbutaline on contractility and K+ uptake in isolated human uterine muscle

Mg++ (3 and 6 mmol/L), the beta 2-adrenergic agonist terbutaline (1 and 10 mumol/L), and dibutyryl cyclic adenosine 5'-monophosphate (0.1 and 1 mmol/L) suppressed spontaneous activity and the increase in contractile activity induced by ouabain or K(+)-free buffer in isolated human pregnant myometrium. The ouabain-suppressible rubidium 86 or potassium 42 uptake was unaffected by the presence of Mg++ (3 and 6 mmol/L), the beta 2-adrenergic agonist terbutaline (1 and 10 mumol/L), or dibutyryl cyclic adenosine 5'-monophosphate (1 mmol/L). However, loading of the strips with Na+ and incubation in high K+ induced a fivefold increase in rubidium 86 uptake. On the basis of these flux rates, our previous data on the total concentration of sodium-potassium pumps in the human myometrium, and an estimated maximum transport rate of the sodium-potassium pump of 8900 K+ ions per minute at 30 degrees C, it could be calculated that the sodium-potassium pump in the Na(+)-loaded strips reached around 80% of its maximal rate. Taken together, these results showed that the relaxant effects of Mg++, terbutaline, and dibutyryl cyclic adenosine 5'-monophosphate on human myometrium are not due to a stimulation of active sodium-potassium transport.

GTP-dependent regulation of myometrial KCa channels incorporated into lipid bilayers

The regulation of calcium-activated K (KCa) channels by a G protein-mediated mechanism was studied. KCa channels were reconstituted in planar lipid bilayers by fusion of membrane vesicles from rat or pig myometrium. The regulatory process was studied by exploring the actions of GTP and GTP gamma S on single channel activity. KCa channels had a conductance of 260 +/- 6 pS (n = 25, +/- SE, 250/50 mM KCl gradient) and were voltage dependent. The open probability (Po) vs. voltage relationships were well fit by a Boltzmann distribution. The slope factor (11 mV) was insensitive to internal Ca2+. The half activation potential (V1/2) was shifted -70 mV by raising internal Ca2+ from pCa 6.2 to pCa 4. Addition of GTP or GTP gamma S activated channel activity only in the presence of Mg2+, a characteristic typical of G protein-mediated mechanisms. The Po increased from 0.18 +/- 0.08 to 0.49 +/- 0.07 (n = 7, 0 mV, pCa 6 to 6.8). The channel was also activated (Po increased from 0.03 to 0.37) in the presence of AMP-PNP, a nonphosphorylating ATP analogue, suggesting a direct G protein gating of KCa channels. Upon nucleotide activation, mean open time increased by a factor of 2.7 +/- 0.7 and mean closed time decreased by 0.2 +/- 0.07 of their initial values (n = 6). Norepinephrine (NE) or isoproterenol potentiated the GTP-mediated activation of KCa channels (Po increased from 0.17 +/- 0.06 to 0.35 +/- 0.07, n = 10). These results suggest that myometrium possesses beta-adrenergic receptors coupled to a GTP-dependent protein that can directly gate KCa channels. Furthermore, KCa channels, beta-adrenergic receptors, and G proteins can be reconstituted in lipid bilayers as a stable, functionally coupled, molecular complex.

Hormonal regulation of potassium currents in single myometrial cells

Three potassium currents (IK) were recorded from myometrial cells isolated from the uterus of rats at estrus and diestrus and kept in culture for 1-6 days. IK were differentiated by their modulation with norepinephrine and/or by their onset kinetics. At +50 mV the activation time constants were about 0.7 ms, 6 ms, and 15 ms for the fast, the intermediate, and the slow IK, respectively. Norepinephrine (1 microM) potentiated the fast IK and reduced the intermediate IK. In addition, differences were found with respect to cells from animals at estrus and diestrus. The fast IK was preferentially expressed in cultures from animals at estrus, whereas the intermediate IK was more frequent in cells from rats at diestrus. These results indicate that K+ channels from myometrial cells are multiregulated. Regulation may occur by short-term signals (neurotransmitters) and/or by preferentially expressing distinct types of channels depending on the hormonal status of the animal.

Augmentation by external Mg ions of beta-adrenoceptor-mediated actions in the longitudinal muscle of rat uterus

1. The longitudinal muscle isolated from the uterus of oestrogen-treated rats was not spontaneously active in Locke solution, and electrical stimulation evoked phasic contraction. Isoprenaline (3 x 10(-11) - 10(-8) M) and dibutyryl cyclic AMP (db cyclic AMP, 0.1-0.8 mM) depressed the phasic contraction; the depression was enhanced in the presence of 0.6 mM Mg. 2. The contracture generated by 40 mM K was partially relaxed by isoprenaline (10(-11) - 10(-8) M) and db cyclic AMP (0.1-0.8 mM). Mg (0.6 mM) enhanced the isoprenaline-induced relaxation, but not that induced by db cyclic AMP. 3. The membrane potential of the muscle was -61 mV, and electrical stimulation induced an action potential which consisted of spike and plateau components. Application of isoprenaline and db cyclic AMP mainly reduced the duration of the plateau potential. The effect was potentiated by 0.6 mM Mg. 4. The membrane was hyperpolarized, accompanied by a decrease in membrane resistance, when 10(-8) M isoprenaline or 0.8 mM db cyclic AMP was applied. The effects of isoprenaline were prominently augmented in the presence of 1.2 mM Mg, while those of db cyclic AMP were slightly potentiated. 5. Forskolin (0.1 microM) or papaverine (10 microM) inhibited the phasic contraction and the K-contracture. The effect on the phasic contraction was potentiated by 0.6 mM Mg, while that on the K-contracture was not affected. 6. Forskolin shortened the action potential at 0.3 microM, and hyperpolarized the membrane with a decrease in membrane resistance at 3.0 microM. The membrane effects were augmented by 0.6 and 1.2 mM Mg, respectively. 7. It was hypothesized that external Mg ions could affect at least two processes involved in actions at beta-adrenoceptors on rat myometrium; receptor-agonist interaction and cyclic AMP-mediated inhibition of membrane excitability.

Involvement of cathepsins B and H in lysosomal degradation of horseradish peroxidase endocytosed by the proximal tubule cells of the rat kidney: II. Immunocytochemical studies using protein A-gold technique applied to conventional and serial sections

Immunoelectron microscopic localizations of cathepsins B and H and injected horseradish peroxidase (HRP) in the lysosomal system of rat kidney proximal tubules were investigated by a protein A-gold technique. Kidneys were fixed by perfusion at fixed intervals after intravenous injection of HRP. At 5 to 15 minutes after the injection, the endocytic apparatus--including the apical vesicles, tubules, and vacuoles (endosomes)--were stained for HRP, but they were negative for cathepsins. Within 15 to 30 minutes after the HRP injection, HRP-containing endosomes were fusing with preexisting lysosomes. In the S1 segment, they accumulated in the apical cytoplasm and formed giant phagosomes, which increased markedly in number and size after 1 hour. These phagosomes were composed of a peripheral clear matrix and electron-dense inclusions. The clear matrix was stained heavily for cathepsins and HRP, whereas the electron-dense inclusions were consistently negative for cathepsins and HRP. The same results also were obtained after the double-labeling and serial sectioning techniques. The dense inclusions were fragmented gradually as the phagosomes decreased in size. After 3 hours, the size and number of phagosomes returned to their normal state (before the HRP injection). These results indicate that the endocytic apparatus of the proximal tubule cells does not contain cathepsins. Phagosomes are formed by the fusion of endosomes containing the internalized protein with the preexisting lysosomes. The degradation of HRP in giant phagosomes occurred rapidly. The coexistence of cathepsins B and H with the endocytosed HRP suggests that these cystein proteinases are involved in the degradation of protein in heterophagosomes of the proximal tubule cells.

Beta-adrenergic actions on membrane electrical properties of dissociated smooth muscle cells

Studies were carried out to determine the effects of the beta-adrenergic agent, isoproterenol (ISO), on membrane electrical properties in single smooth muscle cells enzymatically dispersed from toad stomach. In cells bathed in buffer of physiological composition, the average resting potential was -56.4 +/- 1.4 mV (mean +/- SE, n = 35). The dominant effect of exposure to ISO was hyperpolarization. The hyperpolarization was apparent in all cells studied and averaged 11.6 +/- 1.2 mV (n = 27). In the majority of the cells, hyperpolarization was accompanied by a decreased input resistance (Rin). Often the change in resistance appeared to lag behind the change in membrane potential. The lack of coincident changes in membrane potential and resistance may reflect a superposition of the outward rectification properties of the membrane on beta-adrenergic-induced increases in ionic conductance. In about half of the cells, an initial small depolarization (3.1 +/- 0.3 mV, n = 14) was accompanied by a small but distinct increase in Rin (12 +/- 2.5%). When membrane potential was made more negative than the estimated equilibrium potential for K+ (EK) by injection of current, ISO also produced biphasic effects, an initial hyperpolarization which reversed to a sustained depolarization to a value (-90 mV) near the estimated EK. The hyperpolarization by ISO could be diminished in a time-dependent manner by previous exposure to ouabain. The inhibition by ouabain, however, appeared to be a fortuitous result of glycoside-induced positive shifts in EK. These observations indicate that the dominant electrophysiological effect of beta-adrenergic stimuli is to hyperpolarize the cell membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of ionic currents in smooth muscle balls of the rabbit intestine by intracellularly perfused ATP and cyclic AMP

The effects of intracellularly perfused ATP and cyclic-AMP (c-AMP) on ionic currents recorded from fragmented smooth muscle cells (smooth muscle ball; SMB) were investigated, using the single electrode whole cell voltage clamp method. The Ca2+ current was distinguished from K+ currents, using pipette solution containing Cs+, TEA+ and 4 mM EGTA. ATP enhanced the Ca2+ current dose-dependently between 0.3 and 10 mM, and slightly slowed the slow component of the decay of the Ca2+ current, while the steady-state inactivation curve remained unaffected. Intracellular application of 5'-adenylyl-imidodiphosphate (AMP-PNP; 1 mM) inhibited the Ca2+ current by competition with ATP, but c-AMP (up to 300 microM) had no effect. With a high-K+ solution containing 0.3 mM EGTA and ATP in the pipette and physiological salt solution in the bath, a net inward current with transient (Ca2+ dependent) and delayed (Ca2+ independent) K+ outwart currents were evoked. Increased concentrations of ATP (above 1 mM) but not c-AMP (up to 100 microM) in the pipette enhanced the transient K+ outward current. Neither agent had any effect on the delayed outward current. When repetitive stimulations of intervals shorter than 5 s were applied, the amplitude of the transient outward current was markedly reduced, and 100 microM c-AMP partially prevented this attenuation. ATP may act on the Ca2+ channel either by phosphorylating the channel protein or by other ATP requiring mechanisms, independently from those induced by the action of c-AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic inhibition of bovine mesenteric lymphatics

The beta-action of catecholamines on lymphatic smooth muscle was studied by observing the effect of isoprenaline on electrical and mechanical activity in the double sucrose-gap. Action potentials and phasic contractions evoked by depolarizing pulses were abolished within 2 min of drug addition. Isoprenaline hyperpolarized the membrane and increased membrane conductance. Tetraethylammonium (10 mM) did not itself affect membrane resistance but reduced the hyperpolarization and the increase in conductance caused by isoprenaline. Removal of K+ from the external solution reduced membrane conductance and increased the hyperpolarization due to isoprenaline. When the NaCl content of Krebs solution was replaced with LiCl or choline chloride, isoprenaline no longer blocked action potential firing and its effects on phasic contractions and membrane conductance were reduced. In contrast, ouabain (10(-5) M) did not block the effect of isoprenaline on membrane potential and membrane conductance. These results suggest that beta-adrenergic inhibition of lymphatic smooth muscle involves an increase in an outward K+ current, though an additional metabolic effect cannot be ruled out.

Forskolin effects on longitudinal myometrial strips from the pregnant rat: relationship with membrane potential and cyclic AMP

The effects of forskolin on tension, membrane potential and cyclic AMP accumulation were studied in longitudinal myometrial strips from pregnant rats. 0.1 microM forskolin reduced the amplitude of spontaneous contractions by decreasing the frequency of action potential discharge without a change in resting potential or cyclic AMP accumulation. Forskolin, 1.0 microM, abolished contractions and action potentials, hyperpolarized the membrane and increased cyclic AMP accumulation. Ouabain, 1 mM, depolarized the muscle and increased resting tension. Ouabain reduced potential change produced by forskolin but did not prevent the relaxation or cAMP accumulation. Therefore changes in membrane potential are not prerequisite for the inhibitory actions of forskolin. The cyclic AMP-related relaxation may result primarily from intracellular events that remove calcium from the contractile elements.

Hormonal influence on the response of the uterus of the Brazilian marsupial Didelphis albiventris to adrenergic drugs

The influence of catecholamines on the estrogenized uterus of the Didelphis albiventris was studied in an in vitro preparation. It was observed that the D. albiventris uterus reacts to both alpha and beta-adrenergic agonists but not dopaminergic ones. Adrenaline is more potent in inducing a contractile response than is noradrenaline. Isoprenaline, when added to the bath, is able neither to induce a response nor to interfere with a contraction previously induced by acetylcholine. It is, however, able to block the contractile response brought about by electric field stimulation. In an in vitro preparation from animals previously treated with estrogen and progesterone, the sensitivity to both alpha- and beta-adrenergic drugs was greatly increased. This is demonstrated by a higher pD2 in progesteronized animals compared to that in estrogenized ones. The increased sensitivity to beta-agonists after progesterone treatment is illustrated by the fact that isoprenaline is effective in inducing a relaxation of the uterus, in these conditions. These results indicate that the Brazilian opossum should be included among those animals in which progesterone enhances uterine sensitivity to drugs.

Myometrial responses in situ to nicergoline, acebutolol, phentolamine and noradrenaline of the rat in proestrus

The effects of two adrenoceptor antagonists, nicergoline (alpha 1) and acebutolol (beta 1), on the contraction of myometrium in the proestrous rat were compared to those of noradrenaline and phentolamine. The spontaneous myometrial contractions of Wistar rats on the day of proestrus were recorded isometrically and the data were analysed using Wilcoxon non-parametric statistics. All drugs were administered i.v. and the doses are expressed as microgram/kg body weight. Noradrenaline (1200 micrograms/kg per h) induced a 32.5% reduction (P less than 0.001) of the uterine contraction amplitude. Nicergoline did not alter uterine motility significantly when administered alone at doses ranging from 400 to 1600 micrograms/kg. However, successive injections of nicergoline in the same range given during noradrenaline infusion at 600 micrograms/kg per h potentiated the relaxing action of the latter (38%, P less than 0.01). Phentolamine (120 micrograms/kg) reduced myometrial activity by 25% (P less than 0.05). This inhibitory response rose to 65% (P less than 0.001) when the dose of phentolamine was increased to 960 micrograms/kg. When a single injection of nicergoline (400 micrograms/kg) was followed by the administration of increasing doses of acebutolol (120, 1200, 2400 micrograms/kg) the slight inhibitory effect on uterine motility observed after administration of each of the two agents separately became more pronounced (P less than 0.05). It appears from these results that combining noradrenaline with nicergoline and nicergoline with acebutolol leads to potentiation of their relaxing effects. Furthermore the results confirm that nicergoline is a partial alpha-blocker.

Some properties of the circular myometrium of the sheep throughout pregnancy and during labour

Spontaneous electrical and mechanical activity were measured in strips of circular myometrium, stretched to in vivo length, obtained from ewes during pregnancy and parturition. The resting tension was high (280 mN/mm2) early in pregnancy and decreased gradually during the course of gestation to reach low levels (20 mN/mm2) during labour. Strips showed continuous electrical and mechanical activity up to around day 50 of pregnancy. The variability in the amplitude of the action potentials, the occurrence of many small fluctuations in membrane potential and the short space constant (0.9 mm) could be explained by impaired conduction in these tissues. During the second half of pregnancy, muscle strips showed regular bursts of spike activity and contractions with a frequency of about 4/min. The space constant (lambda) increased gradually to around 2 mm at term. The membrane time constant (tau m) remained constant at approximately 130 ms throughout gestation. Tissues from ewes in labour were not spontaneously active but action potentials could be evoked by depolarizing current pulses. The space constant increased to 4.6 +/- 0.4 mm and tau m increased to 510 +/- 50 ms at this time. Verapamil (10(-6) M) rapidly abolished electrically evoked activity in strips obtained from ewes in labour. Verapamil abolished spontaneous activity in strips from animals up to day 145 of pregnancy, though a higher concentration (2 X 10(-5) M) was required. However, action potentials could be evoked by depolarizing current pulses and these were accompanied by contractions that were 50-80% of those observed in control solution. Cobalt (2mM) had an effect on spontaneous and electrically induced activity that was similar to that of verapamil. The threshold for the initiation of an action potential was increased in all tissues in the presence of the blockers. Verapamil decreased lambda by about 50% in the later stages of pregnancy and during labour. During labour alone verapamil reduced tau m by about 50%. These results show that the onset of labour in the ewe is associated with rapid and dramatic changes in both the passive and active properties of the circular muscle of the uterus.

Potassium supplementation in ritodrine-induced hypokalemia

Acute hypokalemia occurs during infusion of beta 2 agonists for tocolysis. This study examines the efficacy of supplemental potassium in treating this hypokalemia. Four groups of dogs were anesthetized and given lactated Ringer's solution (group I), potassium chloride (group II), ritodrine hydrochloride (group III), and ritodrine plus potassium (group IV). Arterial blood gases, pH, and serum and urinary electrolytes were measured. Results were analyzed by an analysis of variance. Serum potassium fell in groups I and III, rose in group II, and remained stable in group IV. Urinary potassium levels in groups that received ritodrine (III and IV) were not different from control levels. Potassium given with ritodrine will prevent hypokalemia. However, the risks of hyperkalemia exist if vigorous replacement is undertaken. There were no dysrhythmias and no adverse effects in any of the hypokalemic animals. Therefore, the routine administration of potassium is not advocated even in obstetric patients who undergo general anesthesia.

Activity of circular muscle of rat uterus at different times in pregnancy

During pregnancy, circular muscle activity changes from weak irregular contractions at midterm to strong regular contractions at term. The present study was designed to determine if this change is related to contractile ability or to membrane electrical events. Myometrial segments were taken from rats on days 16-17, 18-19, 20, and 21 (term) of gestation. Contractile ability was estimated as the maximum force elicited by 121 mM KCl or 0.6 mM acetylcholine. The K- and acetylcholine-induced contractions were significantly greater than spontaneous and no different at midterm and term. Electrical activity was measured with intracellular electrodes. Between days 16 and 21, action potentials evolved from single, plateau-type to repetitive spike trains. Plateau potentials were associated with small contractions and repetitive spikes with larger contractions. On day 21, there was a transient increase in membrane potential and cessation of spontaneous activity. Before delivery, the membrane potential declined and spontaneous spike trains appeared and triggered large contractions.

CONCLUSION

the change in circular muscle activity during pregnancy is related to electrical events and not to contractile ability.

Electrical responses of smooth muscle cells of the mouse uterus to adenosine triphosphate

Electrical responses of the smooth muscle cells to ATP were recorded in the longitudinal muscle of mouse myometrium, using intracellular micro-electrodes. ATP (greater than 10(-7) M) dose-dependently produced a biphasic change in the membrane potential, an initial hyperpolarization (20-30 sec) and then a depolarization. This effect of ATP was observed in all stages of gestation. The initial hyperpolarization was more quickly desensitized than the depolarization. Application of ATP for a short period (10 sec) produced only the initial hyperpolarization; the amplitude was dose-dependently increased. During the ATP-induced hyperpolarization and the depolarization, generation of spike potentials was suppressed and enhanced, respectively. Strong outward current restored the spike generation during hyperpolarization. During the ATP-induced hyperpolarization, the membrane resistance was decreased. The amplitude of the hyperpolarization was increased in low [K]0 solution and decreased in high [K]0 solutions. Pre-treatment with TEA (1 mM), procaine (1 mM), 4-aminopyridine (0.5 mM) or apamin (2 X 10(-7) M) did not, but TEA (5-10 mM) did suppress the ATP-induced hyperpolarization. Involvement of endogenous catecholamines, cyclic AMP, prostaglandins or acetylcholine in the ATP responses was ruled out. During the ATP-induced depolarization, the membrane resistance was reduced. In low [Na]0 solutions, the muscle membrane was depolarized and the amplitude of ATP-induced depolarization was reduced. In sodium-free solution, ATP produced only the initial hyperpolarization. It was concluded that the electrical responses of the smooth muscle cells of mouse myometrium to ATP consist of two components: an initial hyperpolarization with increase in the potassium conductance and a depolarization with increase in the sodium conductance.

Mechanism of action of vasoactive intestinal polypeptide on myometrial smooth muscle of rabbit and guinea-pig

1. The action of vasoactive intestinal peptide (VIP) on the electrical and mechanical activity of strips of longitudinal myometrial smooth muscle from rabbits and guinea-pigs treated with oestradiol was studied in the sucrose-gap apparatus. 2. In myometrial strips which spontaneously exhibited regular contractions, or which were induced to contract rhythmically to the application of oxytocin, VIP reduced both the frequency and the force of contraction. 3. Contractions were associated with bursts of action potential discharge. In guinea-pig, the membrane potential reached its most negative value shortly following a burst and a slow decay of negativity followed ("generator potential'). VIP inhibited the decay of this negativity and increased the duration of the period between bursts. In rabbit myometrical strips, electrical discharges occurred less regularly but VIP also had an inhibitory action. The inhibitory action of VIP was not affected by the beta-adrenoreceptor blocker propranolol, by tetrodotoxin, or by apamin. 4. Using the double sucrose-gap apparatus, bursts of action potentials and contractions were elicited with depolarizing electrical pulses in the absence of oxytocin. Changes in membrane resistance were also estimated by eliciting hyperpolarizing electrotronic potentials. VIP hyperpolarized the membrane and inhibited contractions as depolarizing pulses now failed to reach threshold for action potential discharge or fewer action potentials were discharged. A small (about 10%) reduction in membrane resistance was freqeuently observed during the hyperpolarization. 5. If a single action potential was elicited in the presence of VIP, the tension generated by the muscle was less than in its absence. 6. In a calcium-free high-potassium (126 mM) solution, readmitting calcium produced contraction; VIP inhibited this contraction. Activation of beta-receptors by means of isoprenaline had a similar effect but unlike isoprenaline the action of VIP was not blocked by propranolol. 7. It is suggested that the primary action of VIP is on the calcium economy of the myometrial smooth muscle cell, possibly to accelerate sequestration and/or extrusion of calcium from the cell. In some way this is associated with inhibition of the generator potential, hyperpolarization, and with a small increase in permeability of the membrane to potassium.

The action of isoprenaline on the smooth muscle of the guinea-pig taenia coli

1. The beta-action of catecholamines on the smooth muscle of guinea-pig taenia coli was investigated by observing the effects of isoprenaline (2 X 10(-8)-7.2 X 10(-6) M) in the presence of an alpha-blocker, phentolamine (6.3 X 10(-6) M). Electrical and mechanical activity were recorded with the double sucrose gap method. Calcium and potassium fluxes were determined using the 45Ca and 42K isotopes. 2. Isoprenaline suppressed spontaneous spike generation, reduced the size of evoked phasic contractions, and caused a small hyperpolarization of the membrane without change in membrane resistance. These effects were abolished by a beta-blocker, propranolol (6.8 X 10(-6) M). 3. The hyperpolarization induced by isoprenaline was smaller in quiescent, nonstimulated muscle than in active, frequently stimulated preparations. It occurred with the same time course as the reduction in the size of evoked phasic contractions. Both effects were dose dependent and reached a maximum at 7.2 X 10(-7) M-isoprenaline. 4. Hyperpolarization by direct current application did not reduce the size of evoked phasic contractions until excitation threshold was reached. In the presence of isoprenaline, repolarization of the membrane to its original level by depolarizing current application did not restore the reduced phasic contractions to their original size. 5. The slopes of the current-voltage relation in the absence and presence of isoprenaline were parallel, confirming the absence of a change in membrane resistance. Isoprenaline also did not affect membrane resistance when applied in the modified ionic environments used. 6. In different external K concentrations (0.60-29.5 mM) the relationship between the size of the electrotonic potential and the magnitude of the isoprenaline-induced hyperpolarization was linear. A similar, direct relation was seen between isoprenaline hyperpolarization and membrane resistance when the latter was increased by lowering external chloride to 13.3 mM. 7. Excess Cao (7.5 mM) hyperpolarized the membrane and reduced membrane resistance, but the hyperpolarization by isoprenaline was larger than in normal solution, being inversely related to the membrane resistance. The hyperpolarization was directly related to the external Ca concentration, suggesting that the magnitude of the response to the beta-action might depend on the cytoplasmic Ca concentration. 8. In low external Na (18 mM-Nao) the beta-action was scarcely affected. Complete replacement of Na with choline increased membrane resistance, muscle tone and phasic contractions; in this condition the effects of isoprenaline were abolished. 9. When the Na pump was blocked by exposure to zero K, to ouabain, or to both simultaneously, isoprenaline remained highly effective. However, prolonged exposure to ouabain abolished the beta-action. 10. Isoprenaline (1.4 X 10(-6) M) increased 45Ca efflux by about 20%, while 45Ca influx was not changed, and 42K efflux remained constant. 11...

Effect of isoproterenol on the isolated pregnant rat myometrium

In pregnant rat myometrium, isoproterenol (10(-8) M) inhibited spontaneous contractions without causing hyperpolarization. Isoproterenol (10(-6) M) relaxed the depolarized muscle without affecting the membrane potential. The presence of 80 mM Na+ did not affect the degree of high-K+ depolarization. It was also without any influence on the effects of isoproterenol on the depolarized uterus. The results are consistent with the concept that hyperpolarization is not a prerequisite for beta-adrenoceptor induced relaxation of uterine smooth muscle.

Role of Ca in isoproterenol-induced increases in cAMP levels in rat uterus

The influence of alteration in the Ca2+ environment of the tissue on isoproterenol-induced increases in cAMP levels and relaxation was studied in rat uterus. In muscles depolarized with 47.5 mM K+ (with or without Na+), the ability of isoproterenol to increase cAMP levels and to produce relaxation was found to be inversely related to external calcium concentration. The pretreatment of the muscle with D600 or EGTA restored the cAMP response to isoproterenol in the depolarized uterus to a level observed in nondepolarized muscle. The study with Ro 20-1724, a phosphodiesterase (PDE) inhibitor indicated that the failure of isoproterenol to elevate cAMP levels in the depolarized uterus could not be related to the activation of PDE by Ca2+. The exposure of rat uterus to a zero-Ca2+ solution accentuated the increases in cAMP levels produced by isoproterenol. These results have raised the question of a possible regulatory role of Ca2+ in beta-adrenoceptor-induced increases in cAMP levels in uterine smooth muscle.

Stretch receptor control of renin release in perfused rat kidney: effect of high perfusate potassium

1. These studies were conducted in the isolated perfused rat kidney to determine the effect of high perfusate K on the renin release induced by low perfusion pressure, renal vasoconstriction and isoprenaline, and to determine whether the magnitude of the K-induced inhibition equalled that observed with renal vasodilation and high perfusion pressure. 2. Raising perfusate K concentration from 4.2 to 56 mM suppressed basal renin release, and the 56 mM-K inhibited the renin release induced by low perfusion pressure (50 mmHg) or phenylephrine (0.83 micrometers). 3. Isoprenaline (0.79 micrometers) induced a marked increase in renin release; but high perfusate K, propranolol (0.28 mM), papaverine (0.39 mM), or high perfusion pressure (150 mmHg) inhibited this effect. 4. It is concluded that high perfusate K has a powerful inhibitory effect on the renin release induced by renal hypotension, vasoconstriction, and isoprenaline infusion, and that this effect may be mimicked by high perfusion pressure or renal vasodilation. A mechanism is proposed whereby these signals may inhibit renin release by depolarizing the juxtaglomerular granular cells.

The effect of intravenous salbutamol upon plasma and urinary potassium during premature labour

Plasma potassium and urinary potassium excretion were measured in eight patients in premature labour treated with salbutamol, and in ten patients at full term in spontaneous labour. There was a significant fall in plasma potassium concentration following salbutamol infusion, but no change during labour in the control groups. Urinary potassium excretion was similar in both groups.

Interactions between the effects of alpha- and beta-adrenoceptor agonists and adenine nucleotides on the membrane potential of cells in guinea-pig liver slices

1 The beta-adrenoceptor agonist isoprenaline normally causes only a small and inconsistent increase in the membrane potential of cells in guinea-pig liver slices, in contrast to the large hyperpolarizations seen with alpha-agonists. However, after a selective alpha-adrenoceptor agonist has been applied, the response to isoprenaline becomes greatly enhanced. 2 Simultaneous application of small doses of an alpha- and beta-agonist produce hyperpolarizations larger than the sum of the responses to each agent alone. 3 These interactions occur with a range of sympathomimetic amines, including some which are not substrates for various processes for the uptake and inactivation of catecholamines. 4 Hyperpolarizations caused by externally applied cyclic adenosine-3',5'-monophosphate (cyclic AMP) also become larger after application of an alpha-agonist. 5 The adenine nucleotides adenosine 5'-diphosphate (ADP) and adenosine 5'-triphosphate (ATP) hyperpolarize guinea-pig liver cells in the dose range 0.1-1.0 mM. This response is not increased after an alpha-agonist. However, ADP and ATP are themselves able to enhance the response to beta-agonists. 6 These interactions between alpha-agonists, beta-agonists and adenine nucleotides seem to involve steps subsequent to receptor activation. Changes in the intracellular actions of cyclic AMP may be concerned.

Inhibitory mechanisms of isoprenaline in the guinea-pig taenia coli

The beta-receptor stimulating effect of isoprenaline on the spontaneous activities of the guinea-pig taenia coli was investigated in the presence of an alpha-receptor blocking agent, phentolamine (2 X 10(-6) M). Substitution of the external Naion with Li, choline, tris-hydroxymethyl aminomethane or Kion greatly reduced the inhibitory effect of isoprenaline. A treatment with ouabain (4 X 10(-6) M) also had effects similar to those of Na removal. The spontaneous activity was transiently suppressed when 5.9 mM K ion was re-admitted into K-free solution, and this suppressing effect was blocked by ouabain and also by reducing Na concentration to 5.9mM. However, isoprenaline retained its effects in K-free solution containing normal Na concentration or 5.9 mM Na. The relaxation by isoprenaline in these solutions was hardly reduced by ouabain (4 X 10(-6) M). These observations suggest that the beta-receptor stimulating effects of isoprenaline are partly mediated through an activation of the Na-pump and partly through some other mechanism which is inactivated when the preparation is deprived of Na or when the membrane is depolarized.

Levels of cyclic AMP and electrical events during inhibition of contractile activity in vascular smooth muscle

The possible relationship between changes in electrical and mechanical activity and in tissue levels of cyclic AMP (cAMP) during relaxation of the myogenical active rat portal vein has been elucidated in response to different inhibitory stimuli. Isoproterenol, 10(-5) M, caused an initial pronounced inhibition of spike discharge and contractions followed by a partial recover over the 15 min exposure period. This inhibitory response was associated with doubling of tissue cAMP after 1 min and a less pronounced increase in later measurements (5 and 15 min). Papaverine, 10(-4)M, caused, in contrast, an inhibition of electrical and mechanical activity which developed gradually over the entire 15 min exposure period. The levels of cAMP at 1,5 and 15 min all exceeded the corresponding values obtained with isoproterenol. The two drugs in combination gave a biphasic inhibitory response associated with a sustained large increase in the level of cAMP. These drug induced inhibitions were thus accompanied by increased levels of cAMP, but the different time courses of the electromechanical responses could not be rlated to corresponding variations in the content of cAMP. Hyperosmolality (+150 mM sucrose) and mechanical vibration (100 Hz) elicited differentiated inhibitions of smooth muscle activity without changes in tissue cAMP. The mechanical effects elicited by isoproterenol, papaverine and hyperosmolality were accompanied by approximately parallel changes in electrical spike discharge. It is concluded that increased levels of cAMP are not indispensible for vascular smooth muscle relaxation, and that the magnitude of pharmacologically induced inhibitions are not always closely related to the total tissue content of the nucleotide.

The relaxing effect of terbutaline on the human uterus during term labor

The uterine and cardiovascular responses to the adrenergic beta2-receptor stimulator terbutaline (TRB) were investigated in 14 patients in normal term labor. TRB, administered intravenously at a rate of 10 to 20 mug per minute, effectively inhibited uterine activity in advanced labor and also expulsion. Intravenous injection of TRB, 250 mug, diminished oxytocin-induced uterine hyperactivity. No serious side effects of the drug were observed; the circulatory effects were minimal, except for a tolerable maternal tachycardia. The mode of action and clinical application of TRB for inhibition of unwanted uterine activity are discussed.