Differences and similarities in the noradrenaline‐ and caffeine‐induced mechanical responses in the rabbit mesenteric artery

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Abstract
The properties of noradrenaline (NA) [norepinephrine]-induced and caffeine-induced contractions in the rabbit mesenteric artery were investigated and compared, using intact and skinned muscles. Na depolarized the membrane and reduced the membrane resistance. Increased concentrations of NA rarely produced oscillatory potential changes yet consistently produced oscillatory contractions which were abolished by pre-treatment with prazosin or Ca-free solution. Application of low concentrations of caffeine hyperpolarized the membrane; high concentrations depolarized the membrane. NA- and caffeine-induced contractions, but not K-induced contraction, could be recorded in Ca-free, EGTA [ethylene glycol-bis-(.beta.-aminoethylether)-N,N''-tetraacetate] containing solution. Repetitive applications of NA or caffeine led to a cessation of contractions. Responses persisted longer with applications of NA than with caffeine. Different mechanisms are involved in the NA- and caffeine-induced activations, but both agents do release Ca stored in the same site. Release of Ca from the store site by NA requires the presence of an intact muscle membrane with adrenoceptors but this is not the case with caffeine. The different mechanical responses to NA or caffeine may also relate to the behavior of Ca; i.e., Ca may be extruded into the extracellular space in the presence of caffeine, while this ion may be extruded and to some extent, re-accumulate into the store site in the presence of NA. The oscillatory contraction evoked by treatment with NA is discussed in relation to the activation of a Ca-induced Ca-release mechanism in the store site.