Ultrastructure and contractures of the pigeon iris striated muscle.

1. The ultrastructure of adult pigeon iris muscle fibres has been described with emphasis on the distribution of the sarcoplasmic reticulum (SR). Contractures due to superfusion with solutions of different [K(+)] (3-150 mM) and acetylcholine (ACh) and their modification by alteration of external [Ca(2+)] and [Mg(2+)] were studied in isolated pigeon iris.2. The arrangement of the contractile myofilaments was like that of vertebrate skeletal fibres. The SR is well developed in the I-band and sparse at the A-band level. Tubular elements (T-system) which form triads with the SR were seen at all levels of the sarcomere though usually adjacent to the A-I junction.3. K(+) contractures developed monotonically to a steady level which was maintained for the duration of the high [K(+)] superfusion. The response to a standard [K(+)] stepwise change was not altered by conditioning the preparation with various [K(+)].4. Decreasing external [Ca(2+)] from 20 mM to Ca(2+)-free (i.e. no Ca(2+) added), enhanced iris contractures at all [K(+)] and in ACh enriched solutions. The K(+) response was abolished when the iris was superfused with Ca(2+) free solution plus EDTA (2 mM) for 45 min. Increasing [Mg(2+)] had little or no effect on iris contracture.5. Reducing external [Ca(2+)] from 3 to 0.3 mM caused a reduction of 3-7 mV in resting membrane potential and an increase from 3 to 10 mM-Ca(2+) caused 3 to 7 mV membrane hyperpolarization. Muscle fibre input resistance was not affected.6. It is concluded that in the pigeon iris, Ca(2+) required for contractile activation is obtained from internal stores, that membrane potential determines the degree of contractile activation and that the maintenance of the contracture is dependent on the failure of the Ca(2+) releasing mechanism to inactive. In addition, it is speculated that because the iris muscle has only sparse SR at the A-band level of the sarcomere, there may be slow Ca(2+) reaccumulation.