The transepithelial potential difference (TEPD) and resistance
(TER) in
the alveoli, and the pressure in the mammary gland were measured in lactating
mice
at different times after they had finished suckling their young. Oxytocin
administration caused a rise in TEPD, the amplitude of which decreased
with time
after cessation of the pups feeding. At 2·5 h after
cessation, oxytocin administration
caused a short-term decrease in TER, but at 5 h there was a two phase reaction:
a
fall in TER followed by a rise. At 20 h there was a one phase
reaction, a rise in TER.
TEPD in the mammary gland alveoli increased from its initial value
of 18±1 mV to
25±1 mV at 2·5 h, and thereafter fell to zero by 15 h
after suckling. The initial value
of TER, 114±1 kΩ, increased to 131±4 kΩ at
2·5 h and then decreased by 5 h after
suckling, but unlike TEPD it did not reach zero. These experiments show
that
extending the period after cessation of suckling does not cause a rise
in
intramammary pressure and that oxytocin injections cause a short-term rise
in milk
pressure. From these results, we propose a hypothesis of reducing ionic
concentration
gradients between intracellular fluid, cytoplasm of the secretory cell
and milk at
extended periods after cessation of suckling, starting at 2·5 h.