Figure S1. Calculation of spontaneous contractile activity in the mouse IAS. Spontaneous contractile activity was composed of tone and phasic contractions. Spontaneous contractions were divided into six components, each depicted with an arrow and an associated number. These included: peak phasic contraction (1), trough phasic contraction (2), slow phasic contractions (3), tone (4), total phasic contraction (5) and total spontaneous contraction (6). Mean values (±SE) for each component in grams of contraction and percentage of the maximum contraction elicited with 60 mmol L−1 KCl are shown in the table below (see also Table 1 in results section). There were no significant differences in spontaneous contractile activity between WT (n = 7) and W/Wv (n = 7) mice.

Figure S2. Spontaneous electrical and contractile patterns in the WT and W/Wv mouse IAS. Similar fast and slow patterns of electrical and contractile activity were observed in WT and W/Wv mice. Shown are examples of two different electrical patterns in WT (A) and W/Wv (B) mice. Slow wave activity was present in all traces, but in the left traces (a) slow wave amplitude was organized into a ‘waxing and waning’ pattern whereas in the right traces (b) slow waves periodically gave rise to spikes. Examples of two different contractile patterns are also shown for WT (C) and W/Wv (D) mice. The level of zero active contraction (determined with 10 μmol L−1 SNP and 1 μmol L−1 nifedipine) is indicated with a dotted line. All muscles exhibited tone and rapid and slower frequency phasic contractions. The slower rhythm was usually organized into discrete oscillations occurring at approximately 2–4 cpm (Ca, Da) whereas in other cases it was less consistent.

Figure S3. Comparison of spontaneous and evoked electrical events in the WT and W/Wv mouse IAS. (A) Superimposed traces from a cell exhibiting a waxing and waning pattern in either the WT (left) or the W/Wv (right) mouse IAS. Smaller amplitude slow waves (black and red) rose slowly to peak, whereas larger slow waves gave rise to a spike (green and blue). The maximum dV/dt (mV/s) for each event is indicated in the corresponding color. (B) Comparison of inhibitory junction potentials (IJPs) elicited with a single shot (ss) of electrical field stimulation (EFS) in the WT (top) and W/Wv (bottom) mouse IAS. IJPs were present in both muscles in the absence of MRS2500 (left). However, following MRS2500 addition, a 7 mV slow IJP was observed in the WT (top, right) but not the W/Wv (bottom, right) mouse IAS. Maximum dV/dts for the numbered depolarizations are: 1: 596 mV/s, 2: 311 mV/s, 3: 473 mV/s, 4: 449 mV/s, 5: 25 mV/s, 6: 260 mV/s, 7: 31 mV/s, 8: 48 mV/s.

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