The cell type composition of a muscle is one of the features characterizing the function of the muscle. Type I fibres are most prevalent in muscles involved in the maintenance of posture, whereas type II fibres are used for movements which require high power output. Many vertebrate locomotor muscles are composed of a mixture of fibre types. Additionally, one muscle fibre may contain several myosin heavy chain types indicating heterogeneity at fibril level . The m. gastrocnemius (whole) used in this study has a high type IIB content whereas both m. rectus femoris and m. gluteus maximus are predominantly composed of types IIA and IID. The results from myosin heavy chain analysis and fibre typing are somewhat different. The identification of four different fibre types using histochemical methods is difficult because of the presence of several types of myosin heavy chains within one single fibre. Therefore, the MHC analysis is more qualitative. The correlation of high density of DHPRs with IIA fibres was stated on the basis of ATPase activity. This was previously also shown by statistical analysis where fibre size was additionally taken into account .
Nifedipine was used as an antagonist in order to specifically block the dihydropyridine receptors. Nifedipine is a dihydropyridine derivative that binds in a specific, stereoselective and saturable fashion to DHPR. The selective calcium channel inhibitor prevents the influx of extracellular calcium through the L-type calcium channel  and also has a clear effect on the contraction activity of the skeletal muscle fibre. The effects of nifedipine on depolarization-induced force responses are inhibitory and dose dependent . As the half life of the blocker is 2 – 6 h, there is enough time to perform reliable measurements.
By blocking the channel with nifedipine, a clear selective decrease in the contraction force of the different muscles studied was observed (Fig. 3). The inhibition percentages of RF and GLU were, however, significantly higher as compared to that of GAS. This data indicates that the same concentration of blocker causes a different response between different muscles. A similar variability is noted when compared to the cell type composition of the muscles. The muscles with high IIA fibre type content have strongly reduced contraction force as a result of addition of a calcium channel blocker. On the other hand, the effect of a blocker on the muscle with a lower IIA type content is much weaker. The findings are attributable to the varying densities of dihydropyridine receptors in muscles. In GAS, the amount of DHPRs is reduced, likely due to the low IIA fibre type content. Thus the blocking effect of a receptor antagonist is weaker as compared to the muscles with a higher IIA content. Furthermore, the response of GAS and GLU to the gradual saturation of DHPRs with nifedipine molecules is different (Fig. 4). Consistently, the inhibition percentage of the contraction force of GAS decreased less as compared to GLU. In addition, a constant level of the contraction force was reached sooner in GAS indicating a complete saturation of the receptors available.
There are several examples of the divergent behaviour of the dihydropyridine receptor in skeletal muscle. First, dihydropyridines are shown to have both stimulatory  and inhibitory  effects on excitation-contraction coupling. Furthermore, unlike in cardiac muscle, calcium release from SR does not require inward current, and as yet is induced by blockade of dihydropyridine receptors . However, it is clear that DHPRs are essential in excitation-contraction coupling since animals with mdg/mdg mutation, which results in a lack of receptor proteins, die at birth because of paralysis of the respiratory muscles . On the other hand, when the α1-subunit is introduced into the nuclei of the dysgenic myotubes, some cells contract upon an electrical stimulation. Despite of this, the recovered influx of calcium ions is not necessary for the contraction since the cells are contracting also even if the current is blocked with cadmium. Hence calcium antagonist drugs seem to have very few pharmacologically relevant actions on skeletal muscle as observed also by Walsh et al . The role of inward current is still an open question although it has been suggested that the current is needed to maintain the calcium stores inside the cell  or for the conditions of activeness of the voltage sensor .
In the present study we report one plausible explanation for the large variation of the results observed in the behaviour of the L-type calcium channels in skeletal muscle. The uneven density distribution of dihydropyridine receptors indicates a difference in E-C coupling machinery between muscle fibre types. In this study, we showed that the difference is also detectable in the contraction forces between muscles of different cell type composition. Although nifedipine specifically blocks the dihydropyridine receptor, there could also be other differences in addition to the density of receptors between the muscles causing the difference in the contraction forces. A larger number of different muscles and developmental stages might clarify the differences in the present results.
Goodman et al  provided the first evidence that E-C coupling characteristics are related to fibre types based on myosin heavy chain. By measuring depolarization-induced force responses of skinned single fibres of rat, they concluded that the optimum force production of a skeletal muscle is related to its MHC isoform composition. Although we found no published data on the E-C coupling phenotype of MHC IIa isoform, the results from previous studies suggest that the parameters describing force and velocity properties of a single muscle fibre are significantly higher in the fast, type IIA and IIB, fibres than in slow, type I fibres.