The influences of extracellular Ca++and verapamil on the spontaneous activity and contractions induced by electrical stimulation were studied in guinea-pig taenia coli.
Spontaneous contractions were recorded with force transducer, and spike action potentials were measured extracellularly by use of suction electrode. Preparations of taenia coli were stimulated for 5 seconds every 1 minute with alternating current (60 Hz, 3.0 V/cm) through plate electrodes located in parallel with the long asix of the preparation. All experiments were performed in Tris-buffered Tyrode solution which was aerated with 100% 02 and kept at 35 °C.
The results obtained were as follows:
1) The amplitude of spontaneous contractions (isometric tension) was maximal at the concentration of 2 mMCa++and above or below 2 mM Ca++ that was reduc-ed in a dose-dependent manner. The frequency of spontaneous contractions, however, was continuously accelerated as extracellular Ca++increased. Verapamil suppressed both frequency and amplitude of spontaneous contractions dose-dependently, and blocked completely at the concentration of 1 mg/l.
2) The frequency between bursts of spike discharge (bursts frequency) increased in parallel withincreases in extracellular Ca++ } , whereas the frequency of spikes in a burst (spikes frequency) was maximally accelerated at the concentration of 2mM Ca++ and above or below 2 mM Ca++ that was attenuated dose-dependently.
3) Effects of Ca++ on the amplitude of spontaneous contractions were closely related to those on the spikes frequency, while bursts frequency determined the contractions frequency.
4) The electrically stimulated contractions were abolished in Ca++free solution, and then increased dose-dependently until 4 mM Ca++. Above 4 mMCa++, isometric contractions decreased. Verapamil depressed the electrically driven contractions dose - dependently.
5) The inhibitory actions of verapamil on spontaneity and on electrically induced contractions were antagonized by the aministration of extra Ca++.
The above results suggested that external Ca++ are essential for both excitation and contraction in guinea-pig taenia coli, and also have a stabilizing effect on the membrane, and the effects of Ca++ on the amplitude of spontaneous contractions result from those on spikes frequency in a burst.
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