Makoto Wakui

Evidence for an increase in membrane conductance during adenosine triphosphate-induced depolarization in the guinea-pig vas deferens

The effects of exogenously applied adenosine triphosphate (ATP) on the smooth muscle of guinea-pig vas deferens were studied with the double sucrose-gap method. ATP evoked a membrane depolarization which was associated with a decrease in the size of electrotonic potentials. Conditioning hyperpolarization induced by current application caused an increase in the magnitude of the ATP-induced depolarization; the larger the conditioning hyperpolarization, the greater the ATP-induced depolarization. These results are discussed with respect to the ionic mechanism of the electrical event in response to ATP in this tissue.

Decrease in membrane conductance induced by noradrenaline in the smooth muscle of guinea-pig vas deferens

The electrical response of the smooth muscle of guinea-pig vas deferens to exogenously applied noradrenaline (NA) was examined using the double sucrose-gap method. NA evoked a depolarization of the smooth muscle membrane which was associated with an increase in the size of electrotonic potentials. A conditioning depolarization of the membrane induced by current application enhanced the size of NA-induced depolarization, whereas a conditioning hyperpolarization reduced it. When a conditioning hyperpolarization of 25 mV in magnitude was applied, the direction of potential change induced by NA was reversed. These results are discussed with respect to the ionic mechanism of the electrical event in response to NA in this tissue.

Ionic dependence of acetylcholine equilibrium potential of acinar cells in mouse submaxillary gland

The ionic dependence of the acetylcholine equilibrium potential (EACh) of acinar cells in which acetylcholine (ACh) induced hyperpolarization under control conditions was investigated using intracellular micro-electrode recording in superfused segments of mouse submaxillary gland. For measurements of EACh two micro-electrodes were inserted into neighbouring communicating cells, direct current was passed through one of these electrodes and EACh was determined by plotting the relation between the size of the ACh potential and the resting potential. ACh was applied by micro-iontophoresis.A complex potential change was induced by ACh when the membrane potential was set at high levels (−50∼ −80 mV). The appearance of complex responses dependend on the external [Na]. A severe reduction in external Na concentration abolished the appearance of complex responses, whereas alterations of external K concentration had no such effect. The results indicate that a depolarizing component separate from the hyperpolarizing component exists even in acinic in which ACh only evokes hyperpolarization under control conditions. Intracellular injection of TEA ions converted the ACh evoked potential change from hyperpolarization to depolarization in acini superfused with solutions containing Na in concentrations between 50 and 135 mM. However, the conversion was never obtained using solutions with low Na concentration (12.5 mM).The mean EACh was −60 mV under normal conditions. EACh was made more negative (5 mV) by a reduction in external Na concentration from 135 to 12.5 mmol·l−1. EACh was influenced by alterations of external K concentration, particularly when combined with reduction in external Na concentration. Alteration of K concentration from 2 to 20 mmol·l−1 shifted EACh to more positive values by about 40 mV. EACh in acini treated with TEA was about −28 mV in control solution (Na: 135 mmol·l−1) and −35 mV in a low Na concentration (50 mmol·l−1).Assuming that the response in submaxillary gland acinar cells to ACh under control condition is composed of two different kinds of potential changes (depolarization and hyperpolarization), the ionic basis of each of the potential changes and a possible explanation for the mechanism of ACh are discussed.

Dual excitatory actions of acetylcholine at the neuromuscular junction in the guinea-pig vas deferens

The action of acetylcholine (ACh) on the smooth muscle of guinea-pig vas deferens was studied using the sucrose-gap method. ACh, when applied at a concentration of 10−6 M, evoked a depolarization of the smooth muscle membrane which was slow in time course (slow depolarization). When ACh was applied at higher concentrations, another depolarization which was fast in time course (fast depolarization) occurred, overlapping the early part of the slow depolarization. The magnitudes of both depolarizations were concentration-dependent on ACh. TTX and adrenergic receptor antagonists had little effect on either depolarizations, while guanethidine and nicotinic receptor antagonists mainly suppressed the fast depolarization. In contrast, atropine suppressed the slow depolarization. The membrane conductance observed by current application, was reduced during the slow depolarization, and the reversal potential of the depolarization was 18.3 mV negative to the resting membrane potential. Whereas, the reversal potential of the fast depolarization was 27.6 mV positive to the resting membrane potential. This reversal potential was quite similar to that of the adenosine triphosphate (ATP)-induced depolarization, previously observed in the same tissue. From these observations, it is suggested that in the guinea-pig vas deferens, ACh acts on nicotinic receptors at the sympathetic postganglionic nerve terminal, causing the release mostly of a non-adrenergic transmitter, probably ATP. In addition, ACh also acts on muscarinic receptors on the smooth muscle membrane, inducing membrane depolarization resulting from a reduction of the membrane conductance to potassium ions.