Recent studies demonstrated different contracture responses in muscle from malignant hyperthermia susceptible (MHS) compared to normal (MHN) individuals following exposure to the plant alkaloid ryanodine in-vitro. To confirm if ryanodine has a specific action in MHS muscle, the effect of a single concentration was investigated in skeletal muscle from MHS, MHN and control subjects using a new evaluation technique. In-vitro contracture test (IVCT) and MH diagnosis were performed according to the European Protocol in 86 patients sent to us for MH diagnostic testing and in 24 controls. Viable fresh muscle bundles were exposed to a single bolus of ryanodine 1.0 microM. Contracture onset time (OTp: defined as the time (min) from administration of ryanodine to the start of a contracture as measured by a contracture exceeding predrug baseline height), and the time to an increase of the baseline height to 10 mN above the predrug level (10Tp) were recorded. 29 patients were identified by IVCT to be MHS, 50 MHN, 7 MHE (equivocal) and 24 controls MHN. The indices from the ryanodine test separated all MHS (OTp: < 16 min; 10Tp < 27.4 min) from MHN (> 18 and > 27.7 min) and control subjects (> 17.4 and > 29 min). Values for MHE (equivocal) individuals ranged from 17.1 to 27.8 min for the OTp and from 32 to 49.2 min for the 10Tp. 5 patients with fulminant MH crises were included in the MHS group and showed the 95% confidence intervals (CI) of the median value < or = 8.05 min (OTp) and < or = 13.35 min (10TP) for MHS. In contrast, CI of the median value for the control group were found to be > or = 25.2 min (OTp) and 43.15 min (10Tp) for normal muscle. Thus the ryanodine test protocol showed markedly different contractures in MHS and MHN or control muscle. These results suggest that MHS muscle has a higher sensitivity to ryanodine. However, the protocol should be investigated for reproducibility and validation of thresholds by other laboratories. Ryanodine can help to improve MH diagnostic tests.