ELECTRICAL
INHIBITION (EI) OF UTERINE CONTRACTIONS DEPENDS ON THE TOTAL DOSE AND NOT
THE FREQUENCY
Karsdon J, Shi S-Q2, Maner WL2, Saade G2,
Garfield RE2
2 The University of Texas Medical Branch, Galveston, TX,
USA
Objective: Uterine
contractions are mechanical events resulting from the underlying electrical
activity of the myometrium. Previous studies showed an electrical current
applied to the uterus inhibits in vitro and in vivo contractility, in a
dose dependant manner. The EI dose, at constant amperage, depends on the
electrical pulse width (PW) and pulse frequency (Hz). Many authors use
various PW and Hz with variable effects. Our aim was to determine, in vivo,
the most important parameter of EI.
Methods: Timed-pregnant
Sprague-Dawley rat on gestational day 19 was studied. 5 stainless steel
electrodes were sutured along one of the bicornuate uterine horns and an
intraperitoneal radiotelemetric transmitter measured intrauterine pressure
(IUP) and electromyographic (EMG) activity. An on-line computer and data
acquisition system collected the transmitter signals and calculated the
intrauterine pressure maximum (IUPmax), area under the contraction curve
(IUPauc) and mean EMG activity (EMGmean). After taking control values for
spontaneous preterm labor contractions EI was started via the electrodes
periodically for 5 seconds (T) with a constant electrical current of square
wave pulses at 5 mA. Two or three recordings were made at each PW e.g.7.5,
15, 30, 60 and 120 ms/pulse and Hz e.g.120, 60, 30, 15 and 7.5 pulses/s. EI
dose, using the root-mean-square (RMS) method, was calculated with the
formula T x mA x [PW/((1/Hz) x 1000)]½ and was kept constant,
i.e. as PW increased Hz decreased.
Results: 5 seconds
of EI significantly decreased IUPmax and IUPauc, more
than 50% of control values. However, there were
no significant differences in IUPmax, IUPauc or EMGmean with variations in
PW and Hz but RMS constant.
Conclusions: EI inhibits
uterine activity in vivo and depends on the total current dose and not only the
frequency. This is important for comparing studies with various parameters
of PW and Hz.