The contractile response to rectangular stimulation by noradrenaline (NA) in rabbit vena cava strips a new method of rapid drug addition

Abstract

A method for rapid addition of a drug was developed in which a predetermined concentration of NA was reached within 0.35 sec. With this method, it is possible to record the mechanical latent period of the contraction as well as the maximum rate of tension development, which occurs a few sec after the beginning of the contractile response. Moreover, this method permits application of a train of drug stimuli. After addition of NA the following essential results have been obtained: (1) latency of 9 vena cava strips decreases from a mean of 4.93 sec (S.D..±0.93) at a NA concentration of 5×10 −9 g/ml, to a mean of 0.91 sec (S.D. ± 0.35) at a NA concentration of 10 −5 g/ml in Ca 2+ -containing solution; (2) the maximum rate of tension development increases linearly with the intensity of NA concentration up to 5×10 −7 g/ml; (3) latent period and maximum rate of tension development are significantly altered, when the strips are incubated in Ca 2+-free solution; (4) latent period, maximum rate of tension development and height of the maximum of force after shortlasting NA addition (5×10 −9 g/ml) depend on the interval between drug stimuli, when a static value is attained. The findings indicate that the latent period in not only due to different diffusion times of NA but depends mainly on the time required for excitation-contraction coupling, while the maximum rate of tension development serves as a suitable parameter for describing the fast component of responses to NA even in contractions, in which the first component merges without any intervening contraction with the second component.