In vivo pH and metabolite changes during a single contraction in rat uterine smooth muscle

Abstract

1. We have used 31P NMR spectroscopy to measure metabolites and pHi at three periods during a phasic contraction of the uterus, in vivo, to determine whether they change as a consequence of contraction. The regular uterine contractions were recorded via a balloon catheter in the uterine lumen. Each phasic contraction was divided into three parts: the period between contractions (rest), the development of force (up) and the relaxation of force (down). The NMR data were summed separately from each of these three periods over 20-40 successive contractions. 2. Significant changes in ATP, phosphocreatine (PCr) and inorganic phosphate (Pi) occurred during the contraction. [ATP] fell from 2.0 to 1.6 mM and [PCr] from 2.6 to 2.0 mM during the up period, while [Pi] increased from 2.2 to 2.8 mM. Recovery of ATP and PCr occurred during the relaxation part of the contraction, whereas Pi did not fully recover until the contraction was complete. 3. Significant acidification from pH 7.28 +/- 0.02 at rest to 7.16 +/- 0.02, occurred with contraction. This acidification is greater than that previously reported for in vitro uterine preparations. Measurements of uterine blood flow show that it decreased with contraction. Therefore, ischaemia, in addition to the metabolic consequences of contraction, may account for the larger acidification observed in vivo. 4. Lowering pHi in an in vitro uterine preparation by a similar level to that found in vivo produced a significant reduction of the phasic contractions. Thus we propose that these changes, especially the fall in pHi during force development, feed back negatively on the contraction to limit its strength, and may help prevent uterine ischaemia and fetal hypoxia during labour.

Figure 1. Uterine contraction and associated NMR spectra

A, recording of in vivo uterine contractions showing how each contraction cycle was divided into three parts: rest, upstroke (up) and downstroke (down). B, ³¹P NMR spectra from the uterus in vivo, obtained via a small surface coil put over the area of the uterus containing the pressure-recording balloon. The spectra were obtained during the three parts of the contraction described in A. Peaks from ATP, PCr and P_i can be seen. The inset shows the spectral region around P_i, to show the acidic shift in pH, i.e. rightward shift, which occurred with contraction. p.p.m., parts per million.

Figure 2. In vivo changes in the concentrations of ATP, PCr and P_i, and in pH_i during a uterine contraction

Mean values ±s.e.m. from 17 animals. The metabolite values are expressed relative to those found over the entire contraction cycle (100%), via ³¹P NMR spectroscopy. * Significantly different from the value at rest.

Figure 3. Changes of blood flow and metabolites in the uterus

A, simultaneous recording of uterine contractions and blood flow, measured via an intrauterine balloon and a laser Doppler flow probe on the uterine artery. The blood flow baseline (0%) was obtained at the end of the experiment, when the rat was dead. B, the mean effects on metabolites and pH_i of occlusion of the uterine artery over the range 20-50% (no occlusion = 100%, n = 9). * Significantly different from control value.

Figure 4. The effect on contractions of arterial occlusion and reduced pH_i

A, the effect of occlusion on uterine force production in vivo. B, using the same tissue studied in A, the effect of acidification on uterine force production in vitro was determined by adding 10 mM butyrate to the perfusate. This provided a mean change of pH_i of 0.12 ± 0.02 pH units (n = 5; mean resting pH_i, 7.11 ± 0.13) and was associated with a significant decrease in force. The effects of pH and occlusion were fully reversible (not shown). pH_i values were obtained using SNARF, as described in Taggart & Wray (1993).