Postnatal Ontogenetic Development of Neurogenic and Myogenic Control in the Rat Portal Vein

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Abstract
The postnatal ontogenetic development of neuro‐effector control in vascular smooth muscle of the singleunit type has been studied in the rat portal vein. Contractile activity was recorded isometrically in isolated preparations from rats 2–38 days of age and in adults rats. Spontaneous activity, characteristic of the adult portal vein, appeared abruptly during the third postnatal week. Whereas, induced responses to noradrenaline (NA) and acetylcholine (Ach) appeared early during the first week and responses to transmural nerve stimulation occurred at the end of the first week. The appearance of spontaneous activity was accompanied by significant increases in sensitivity to NA (log Ed50) and to transmural nerve stimulation (frequency giving half maximum response), but not to Ach. Also, maximum responses for NA and nerve stimulation relative to Ach responses tended to increase during the first there weeks. It is concluded that the development of a mechanism supporting myogenic propagation as revealed by highly synchronized spontaneous contractions is an important factor for promoting the effectiveness of sympathico‐adrenergic control in this type of vascular smooth muscle.