The transition of myometrium from relative quiescence throughout pregnancy to powerful contractions during labour is a very complex and precisely timed process. This process of activation occurs in preterm as well as term labour irrespective of the aetiology. An increase in resting membrane potential, contraction frequency, intercellular coupling through gap-junctions and paracrine interactions is thought to be important for synchronising the activity of uterine myocytes at the end of pregnancy, yet the mechanism of myometrial autorhythmicity remains unclear. An increase in intracellular concentration of Ca2+ ([Ca2+]i) initiates contraction in all types of smooth muscle cells including uterine myocytes. On a tissue level, temporal and spatial summation of [Ca2+]i transients in individual cells triggers contraction patterns specific to particular types of smooth muscle. The mechanisms responsible for the initiation and spread of tissue-level Ca2+ signals differ in different smooth muscles, involving in some cases, a specialised type of cell called interstitial cells of Cajal (ICCs). Recent work has established that ICC-like cells are present in both rodent and human myometrium, although they are unlikely to be the pacemakers, at least in the rat myometrium [1]. We have used laser scanning confocal microscopy to investigate the initiation and propagation of Ca2+ signals between uterine myocytes in their natural environment (i.e. within thin slices of intact myometrium). We observed synchronous and large rises in [Ca2+]i elicited by action potentials in bundles of smooth muscle cells. This was followed by non-propagating asynchronous Ca transients of smaller amplitude, presumably due to spontaneous Ca2+ release from the sarcoplasmic reticulum. Immediately preceding the high-amplitude [Ca2+]i transient in the smooth muscle cell bundle, there were spikes of [Ca2+]i originating outside the bundle, in the interstitial space and propagating towards the muscle bundle. Immunohistochemistry of fixed myometrial slices revealed vimentin-positive ICC-like cells residing within the interstitium and surrounding the smooth muscle cell bundles. Communication of the tissue level action potential is quickly lost during cell culture with spontaneous activity occurring only within the first day of culture. Upon loss on intercellular communication individual cells that spontaneously oscillate are revealed, as well as cells which respond differently to agonists.We further demonstrate that within the population of smooth muscle cells expression of important contraction associated proteins is not uniform. Specifically we show that myometrial smooth muscle cells express the Cav3.1(alpha1G) subunit of the T-type calcium channel in approximately 50% of cells, which is in contrast to the L-type channel being expressed in 100% of cells. In current clamp experiments we observed low voltage activated (LVA) spikes that could be elicited after short hyperpolarizing pulses. These LVA spikes were blocked by 100μM Nickel. We determined that cells that express both LVA and HVA current demonstrate an extended “window current” that may allow for slow depolarization when resting membrane potential approaches -55mV. We therefore hypothesise that T-type mediated calcium entry may act a pacemaker current by setting contraction frequency when resting membrane potential increases during myometrial activation. In agreement with this hypothesis 100 μM nickel produced a four fold decrease in contraction frequency, an effect that was reversible on wash out. The implications of cellular heterogeneity in the process of activation and the genesis of spontaneous rhythmicity will be discussed.Acknowledgements: AB is supported by a Research Councils United Kingdom fellowship.Reference 1 : R.A. Duquette, A. Shmygol, C.Vaillant, A. Mobasheri, M. Pope, T. Burdyga & Susan Wray (2005). Vimentin-positive, c-kit-negative interstitial cells in human and rat uterus; a role for pacemaking? Biology of Reproduction 72, 2, 276-283.