Effects of vasodilators on the isolated taenia coli of the guinea-pig

Calcium dependence of rhythmic contractions of the Oryzias latipes blastoderm

1. The blastoderm of the Oryzias latipes (medaka, Teleostei) embryo begins to contract rhythmically, about once per min at 25 degrees C, during epiboly. When the blastoderm was mechanically detached from the rest of the egg, it contracted into a pear-shaped ball and also continued to contract rhythmically. 2. The optimal [Ca2+] for the rhythmic contractions was approximately 1 mM. 3. The contractions stopped in media containing La3+, Ni2+, Mn2+, Co2+ or Ba2+. 4. A number of organic calcium antagonists--cinnarizine, D600, diltiazem, nifedipine, TMB-8 and verapamil--had no apparent effect on the contractions. However, the contractions were inhibited by papaverine, caffeine, and a mixture of TMB-8 and verapamil. 5. The contractions stopped in a medium containing 25 mM K+ or cytochalasin D. 6. We conclude that microfilaments cause the contractions, that each rhythmic contraction is preceded or accompanied by an increase in cytoplasmic free [Ca2+], and that Ca2+ enters the cytoplasm from both an extracellular and an intracellular pool.

The control of pigment migration in isolated erythrophores of Holocentrus ascensionis (Osbeck). II. The role of calcium

The integumental pigment cells (erythrophores) of the squirrel fish, Holocentrus ascensionis, are specialized for rapid radial transport of the pigment granules contained within their cytoplasm. Pigment granules in isolated denervated erythrophores alternate spontaneously between a centrally aggregated state and a radially dispersed state. In the absence of external calcium, pigment aggregation does not occur spontaneously and cannot be induced by the aggregating agents epinephrine or high concentration of external K+. Pigment aggregation is also impaired in the presence of D600 or papaverine, compounds reported to antagonize calcium influx into the cell. Pigment aggregation can be induced by experimental elevation of the concentration of cytoplasmic free Ca2+, with a Ca-EGTA buffer system in conjunction with ionophore A23187. The threshold concentration of Ca2+ required to produce this effect is 5 X 10(-6) M. These results suggest that cytoplasmic free Ca2+ is involved in mediating pigment aggregation and that some, if not all, the Ca2+ is supplied by influx from the extracellular space.

Calcium dependence and contraction in somite formation

The existence of a calcium-dependent contractile process in the formation of somites from segmental plate mesoderm was investigated using a Ca2+ agonist and Ca2+ and calmodulin antagonists. The contribution of cell movement and apical constriction in the segmentation process were assessed using SEM of normal and drug-treated somite and segmental plate tissue. Explants that contained segmental plates of stage 14-15 chick embryos were cultured on vitelline membranes in calcium- and magnesium-free (CMF) Hands' solution, liquid culture medium, and medium containing drugs. Ca2+ ionophore A23187 promoted the rapid completion of one new somite pair. CMF halted segmentation. The Ca2+ antagonists verapamil and papaverine reversibly inhibited segmentation. Theophylline did not inhibit segmentation, suggesting that the effects of the Ca2+ antagonists are not due to inhibition of phosphodiesterase activity. These results suggest that somitogenesis is Ca2+-dependent. Two drugs that inhibit the binding of calmodulin, chlorpromazine and trifluoperazine (TFP), halted segmentation. The inhibitory effect of TFP was reversible. The effects of TFP on somites were compared with those of cytochalasin D. The contribution of microtubules to cell shape and movement in somitogenesis was examined by incubation with nocodazole, a reversible inhibitor of tubulin polymerization. Cell elongation and somitogenesis were inhibited.

Inhibition of fundic strips from guinea-pig stomach: the effect of theophylline on responses to adenosine, ATP and intramural nerve stimulation

The effect of theophylline on the ATP response, the adenosine response and the inhibitory junction potential was studied on circular smooth muscle preparations of the guinea-pig stomach. The amplitude of the inhibitory junction potential evoked after stimulation of the non-cholinergic, non-adrenergic nervous system was not affected by a moderate concentration of theophylline (5 x 10(-6)-10(-5) M). At higher concentrations (5 x 10(-5)-10(-3) M) theophylline relaxed the muscle hyperpolarized the cell membrane and reduced the inhibitory junction potential slightly. ATP and adenosine (5 x 10(-6)-10(-3) M) also caused relaxation of the smooth muscle cells and hyperpolarization of the cell membrane. Theophylline (5 x 10(-6)-10(-3) M) did not antagonize these effects; in the presence of theophylline (5 x 10(-5)-10(-3) M) additional relaxations produced by ATP and adenosine were limited in view of the muscle tone. These results indicate that theophylline does not inhibit either the effect of the non-adrenergic inhibitory transmitter on the smooth muscle cells of the guinea-pig stomach or the actions of ATP and adenosine. This suggests that the existence of theophylline-sensitive adenosine receptors in the stomach-muscle cell membrane is unlikely and that theophylline is not the drug of choice to support the purinergic nerve hypothesis.

Calcium in epithelial cell contraction

Epithelial morphogenesis in many organs involves asymmetric microfilament-mediated cellular contractions. Similar contractions, in terms of ultrastructure and cytochalasin B sensitivity, can be induced in the carcinoma cell line C-4II in culture. This line was used to compare total intracellular calcium levels ([Ca]i) in contracted monolayer fragments and in control cultures, and to determine whether epithelial cell contraction depends on influx of extracellular Ca. [Ca]i, defined as Ca not displaceable by lanthanum, was measured by atomic absorption spectrophotometry. Degrees of contraction were determined from shape changes of monolayer fragments. Detachment from the growth surface initiated cellular contractions and caused an immediate increase in [Ca]i, from 1.0 to 4.0-5.0 micrograms Ca/mg protein in early confluent cultures, and from 0.3 to 1.0-2.0 micrograms Ca/mg protein in crowded cultures. This increase was followed by a gradual decline in [Ca]i, though Ca levels remained higher than in controls and contraction progressed for 30 min. Contraction was inhibited completely by cold (7 degrees C) and by Ca-free medium, and in a dose-dependent manner by papaverine (2.5 x 10(-6) M-2.5 x 10(-4) M), lanthanum (1.0 x 10(-6) M-1.0 x 10(-4) M); and D-600 (1.0-2.0 x 10(-4) M). The Ca ionophore A23187 had no effect at 5.0 x 10(-6) M and was inhibitory at higher concentrations. The results provided direct evidence for increased [Ca]i in contracting epithelial cells, and suggest that Ca influx is required for such contraction to take place.

Inhibition of fundic strips from guinea-pig stomach: the effect of theophylline on the membrane potential, muscle contraction and ion fluxes

The effect of theophylline on the smooth muscle cells of the fundic part of the stomach of the guinea pig was investigated. Theophylline hyperpolarized the membrane, inhibited spike discharges and slow waves and produced relaxation of the muscle cells. Furthermore, the theophylline-induced relaxation was not affected in low sodium solution or during inhibition of the sodium pump. Partial inhibition of the theophylline relaxation was seen in muscle depolarized by a high potassium solution and also when ATP was the relaxant. The potassium efflux was enhanced by ATP, but was not markedly changed by theophylline. A change in calcium efflux in the presence of theophylline could not be measured in quiescent preparations. The calcium influx was not changed by theophylline in Krebs, by low Ca, or by high K solution. Furthermore, the tissue content of cyclic AMP was increased by 59.6% in the presence of theophylline (2 x 10(-3) M). The conclusion is reached that the most likely mode of action of theophylline is a suppression of spontaneous activity and an extensive calcium binding to internal sites.

A scanning electron microscopic and flame spectrometry study on the role of Ca2+ in amphibian neurulation using papaverine inhibition and ionophore induction of morphogenetic movement

SEM observations reveal that papaverine, which interferes with Ca2+ flux, inhibits neural fold formation causing a flattening of the cellular surface and a broadening of cellular junctions. Ionophore A23187 and EGTA both counter this effect promoting a rapid cellular constriction which results in the formation of neural ridges and folds as individual cells become uplifted and ruffled. Flame spectrometry data indicate that as Ambystoma maculatum or mexicanum embryos neurulate Ca2+ is released to the medium, a condition which is impeded by papaverine. Ionophore A23187 induces Ca2+ influx whereas EGTA induces an efflux. Since both agents affect similar morphological changes it is suggested that the availability of free Ca2+ is crucial in controlling the morphogenetic movements of neurulation. It is now apparent that neuro-axial development in the urodele is accompanied by a Ca+2 efflux, but the mechanism(s) responsible for the ion's release is unknown.

A comparison of the effects of adenosine triphosphate with noradrenaline and with the inhibitory potential of the guinea-pig taenia coli

1. The response of the smooth muscle of the guinea-pig taenia coli to adenosine triphosphate (ATP) and related substances was compared with that to noradrenaline and dopamine. The effect of these substances on the inhibitory potential was investigated.2. ATP in concentration of 10(-6)-10(-5)M suppressed the spontaneous spike and relaxed the preparation. ATP in a concentration higher than 10(-4)M hyperpolarized the membrane, reduced the membrane resistance and decreased the inhibitory potential.3. Effects of noradrenaline (5 x 10(-6)M) and of dopamine (5 x 10(-4)M) were similar to those of ATP (10(-3)M). However, the effects of catecholamines were reduced in Cl-deficient solution and blocked by phentolamine (3.5 x 10(-6)M), while the effects of ATP were unaffected.4. A high concentration of phentolamine (3.5 x 10(-4)M) almost abolished the inhibitory potential as well as the effects of ATP. The inhibitory potential and the effect of high concentration of ATP (10(-3)M) were not impaired by imidazole (50 mM).5. Adenosine diphosphate, adenosine monophosphate and adenosine produced effects essentially similar to those of ATP. However, their potency was less compared with ATP.6. These electrophysiological observations support the hypothesis that ATP may be the transmitter of the non-adrenergic inhibitory nerves in the taenia coli. No evidence against the hypothesis was obtained.