Regulation of the contractile activities of uterine smooth muscle underpins the fundamental physiological processes of parturition. It is accepted that contractions in uterine smooth muscle cells are determined by episodic, spontaneous electrical potentials. However, the mechanisms that spread the electrical excitations and thus contractions throughout the uterus are still in debate [1]. Optical mapping techniques enable the visualisation and quantification of the spatiotemporal spread of electrical excitations with high spatial and temporal resolutions, and at the same time, also allow simultaneous observations of both electrical activities and the corresponding contractions. We, therefore, have undertaken optical mapping experiments to examine the spatiotemporal characteristics of excitation propagation in isolated uterine tissues from pregnant guinea pigs at term.

Optical imaging was performed according to well-established methods described for the heart [2] with some modifications. Uterine tissues marked with position markers were mounted and stretched to its in vivo dimensions then superfused in Krebs (with 95% O2 and 5% CO2) at 37°C. After a period of stabilization, the tissues were stained with the voltage-sensing fluorescent dye di-4-ANEPPS. Fluorescence was excited by light-emitting diodes at 470 nm, and two ranges of emission signals (515 to 565 nm and >590 nm) were projected onto the same image frame and collected simultaneously by a highspeed camera at 250 Hz. A viewing area of 2 cm x 4 cm of uterine tissue was projected onto half of the image frame of 64×128 pixels, resulting in spatial resolutions of 312.5 x 312.5 µm2. Uterine movements were separated from the electrical excitation signals by taking the ratio of the two emission signals [3]. The electrical excitation signals were then subjected to spatial and temporal filtering to reduce noise. Excitation wavefronts and conduction velocity vectors were computed using a modified method based on fitting a polynomial surface at each pixel along the wavefronts [4].

All isolated uterine tissues contracted spontaneously. The underlying electrical activities often originated from all around the edges of the tissue. The propagations consisted of mostly repetitive plane waves with occasional re-entrants that rotated around portions of contracted tissues. Multiple initiations might coexist and these excitations would collide, merge or dissipate, complicating the organisation. However, we never observed chaotic excitation patterns such as those of ventricular fibrillation in the heart. The average conduction velocity of 354 excitations is 4.29+/-1.56 cm/s (mean+/-SD). However, instantaneous speeds of the wavefronts can reach >52 cm/s momentarily, especially when multiple waves merged. The uterine contractions of the whole tissue area did not require complete synchronised excitations, and it can be sustained by excitations originated from multiple directions. Further investigations would advance our understanding of the physiological processes of uterine electrical excitation at tissue and organ levels of organisation.