Proceedings of The Physiological Society

University of Cambridge (2004) J Physiol 555P, SA7

Research Symposium

Voltage control of Ca2+ signalling via G-protein-coupled receptors

Martyn P. Mahaut-Smith

Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK

G-protein-coupled receptors (GPCRs) form the largest family of surface membrane receptors and are not normally considered to be voltage-dependent. However, in certain tissues there is significant evidence to suggest that IP3-dependent Ca2+ release during activation of GPCRs can be modified by changes in the cell membrane potential via a mechanism independent of Ca2+ influx. This phenomenon was initially described in coronary artery smooth muscle cells during activation of muscarinic cholinergic receptors (Ganitkevich & Isenberg, 1993) but has been most extensively characterised during activation of P2Y receptors in the megakaryocyte (MK) (Mahaut-Smith et al. 1999). This non-excitable cell type is the precursor of blood platelets and lacks both voltage-gated Ca2+ influx and ryanodine receptors. In unstimulated MKs, changes in membrane potential have little effect on [Ca2+]i, however during stimulation of P2Y receptors with ADP, depolarisations potentiate and hyperpolarisations inhibit the Ca2+ response. These voltage-dependent [Ca2+]i changes are due to modification of IP3-dependent Ca2+ release (along with associated store-dependent Ca2+ influx) as they are still observed in Ca2+-free or Na+-free media and are abolished by inhibitors of IP3 receptors. The mechanism underlying voltage control of P2Y receptor-evoked Ca2+ signals is unknown, and can be explained by one of three theories: 1. The receptor, its G-protein or phospholipase-C is directly voltage-dependent; 2. The binding of polar agonists or substrates (eg. the agonist or PIP2) is altered by membrane voltage; and 3. A voltage-sensitive protein in the plasma membrane is configurationally coupled to IP3 receptors on the internal stores. Theories 1 & 2 would suggest that IP3 production can be voltage-dependent, which is currently the working hypothesis since depolarisation evokes Ca2+ waves that are indistinguishable from those observed with ADP (Thomas et al. 2001). In addition, a pronounced delay of several hundred milliseconds exists between large amplitude depolarisations and the first detectable Ca2+ increase, indicative of an event involving a diffusion-limited step. Although we have shown voltage dependence to several types of GPCR in the MK, the response is particularly robust during stimulation of P2Y receptors. Consequently, physiological voltage waveforms can markedly regulate ADP-evoked Ca2+ increases (Mason et al. 2000; Martinez-Pinna et al. 2003). The reasons for the robust nature of the depolarisation-dependence to P2Y receptor Ca2+ signalling in the MK is unknown. One explanation is that the P2Y receptors in this cell type (believed to be both P2Y1 and P2Y12 receptors as reported for the platelet) are especially voltage-dependent. Alternatively, the extensive membrane invaginations of the MK (Mahaut-Smith et al. 2003) may enhance an innate voltage dependence to GPCR signalling, for example by increasing the ratio of surface membrane to cytoplasmic volume. Thus, the mechanism may have more relevance in cells with high specific membrane capacitances or in structures with high surface area: volume such as dendritic spines. Indeed, a role can be proposed for this phenomenon in synaptic integration by way of co-incidence detection, since membrane depolarisations can markedly potentiate the [Ca2+]i responses to low concentrations of ADP (Guring & Mahaut-Smith, 2003).

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