Spread of contraction in rabbit ear artery preparations in response to stimulation by norepinephrine.

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Abstract
To investigate the spread of contraction in the rabbit ear artery, changes in segment diameter were measured after local stimulation. No spread of contraction was produced by direct current or local application of norepinephrine (NE). Repetitive electrical stimulation caused contraction which spread far from the point of excitation. Since 2 x 10(-7) g/ml tetrodotoxin (TTX) or 10(-5) M phentolamine prevented this spread of contraction, we concluded that it depends on normal function of periarterial nerves and of the alpha-adrenergic receptors at the site of the smooth muscle cells. To check this conclusion and exclude the possibility of myogenic propagation, which normally is due to the conduction of action potentials, the relation between membrane potential of smooth muscle cells and the rapid phase of the contractile response after rapid addition of NE was investigated. In both polarized and depolarized tissues NE induces a biphasic contractile response with no difference in latency. Membrane potential does not change during mechanical latency after the rapid addition of 5 x 10(-9) - 5 x 10(-6) g/ml NE. There are only slight differences in contraction amplitude and maximum rate of tension development when strips from polarized and depolarized tissues incubated in Ca2+-free solution are stimulated by NE. These findings support the conclusion that spread of contraction is not due to myogenic propagation.