Contractile activation characteristics of single permeabilized fibres fromlevator palpebrae superioris, orbicularis oculi and vastus lateralis muscles from humans

1.. We investigated the contractile activation characteristics of single me mbrane-permeabilized fibres from the following muscles from humans: the lev ator palpebrae superioris (LPS), an extraocular muscle; the orbicularis ocu li (OO), a facial muscle; and the vastus lateralis (VL), a major muscle of the thigh. 2. Single permeabilized muscle fibres were isolated from each of the differ ent muscles, attached to a sensitive force transducer and activated by rapi d immersion in buffered solutions of varying [Ca2+] and [Sr2+]. Fibres were allocated into discrete populations based on their contractile characteris tics, including their differential force responses during Ca2+ and Sr2+ act ivation. 3. With the exception of one fibre from the LPS, all 152 fibres sampled fro m the three different human muscles could be classified into either populat ion I(slow, type I) or population II (fast, type II) based on their force-p Ca(pSr) relations. The LPS muscle fibre which was unable to be classified i nto the two major fibre populations displayed a combination of the typical. force-pCa(pSr) relations for mammalian fast and slow muscle fibres. 4. Although fibres from the LPS, OO and VL muscles had similar differential sensitivities to Ca2+ and Sr2+, the steepness of the force-pCa(pSr) curves for fibres from the LPS and OO muscles were highly variable compared with those for fibres from the VL muscle. Specific forces (N cm(-2)) of the smal ler diameter fibres from the LPS and OO muscles were significantly lower th an those of fibres from the VL muscle. 5. The differences in the contractile activation characteristics between fi bres from the VL muscle and those of fibres from facial (OO) muscles and ex traocular (LPS) muscles, reflect the differences in their fibre composition that are responsible for their functional specificity.