The differences in contractile response to AC field stimulation between longitudinal and circular muscles of guinea...

Contractile response to AC field stimulation was compared between the longitudinal and circular muscles of the guinea pig stomach under various stimulating conditions. The longitudinal muscle responded with considerable tension at any frequency between 20 and 2,000 Hz. The circular muscle showed a peak response around 200 Hz with rapidly decreasing response to the higher frequencies of less than 800 Hz. Experiments in the presence of tetrodotoxin and atropine revealed that the circular muscle was scarcely stimulated directly (without nerve excitation) by the AC, while the longitudinal muscle was stimulated directly at frequencies higher than 400 Hz, as well as being stimulated indirectly at frequencies lower than 200 Hz. The field strength necessary to generate one-half maximum tension was higher in the circular muscle (2.8 V/cm) than in the longitudinal muscle (1.5 V/cm at 200 Hz), although when a frequency lower or higher than 200 Hz was applied, the strength-tension curve shifted to the left or right, respectively. The contractile responses of the two muscles were graded with respect to filed duration up to maximum tension. The maximum tension and the highest rate of tension development were given at the stimulating condition of 200 Hz, 5 V/cm, for 5 sec in both types of muscles. The maximum tension generated per unit cross-sectional area of the longitudinal muscle was 19.2 g/mm2 and that of the circular muscle was 32.5 g/mm2. The maximum rates of tension development of the longitudinal and circular muscles wer 9.1 and 12.1 g/(mm2 . sec), respectively.