This paper addresses a new prioritized multichannel multi-time slot MAC protocol (PMCMTP) for large-scale WSNs especially for Ultra-Wide Band (UWB) based networks. To reduce the complexity of resource sharing, the global network is composed of a set of Personal Area Networks (PANs) or cells. According to available resource and PANs duty cycle, PMCMTP can dynamically assign several data channels per PAN and efficiently allocate time slots to each PAN's members. This significantly decreases delay and increases throughput. Through some simulations, we evaluate the performance of the proposed protocol. The results show that PMCMTP ensures an efficient and fair channels allocation between cells permitting, on the one hand, an enhancement of quality-of-service inside each PAN and, on the other hand, a maximization of channel utility.