Connexin43 (Cx43)-formed hemichannel-like structures at the inner mitochondrial membrane are central for infarct size reduction by ischemic and pharmacological preconditioning, and loss of mitochondrial Cx43 (mCx43) abrogates cardioprotection. The open probability of Cx43-formed hemichannels is regulated by nitrosylation in astrocytes and endothelial cells. Conversely, Cx43 interferes with the expression of nitric oxide synthase (NOS) isoforms, indicating a complex interplay between Cx43 and NOS-derived nitric oxide (NO).

Objective: 

Since mitochondria are supposed to contain a NOS, we now investigated the expression of NOS isoforms and the NO-synthesis rate in isolated mitochondria of wild type (WT) and Cx43-deficient (Cx43^cre/fl) mice hearts.

Methods: 

Mitochondria were isolated from mice hearts using differential centrifugation and purified via percoll gradient ultracentrifugation. Isolated mitochondria were stained with an antibody against a mitochondrial marker protein, (adenine-nucleotide-translocator; ANT) in combination with either a neuronal NOS (nNOS) or an inducible NOS (iNOS) antibody and analyzed using confocal laser scan microscopy. The NO-synthesis rate was quantified in purified heart mitochondria using the oxyhemoglobin (oxyHb) assay.

Results: 

A co-localisation of predominantly nNOS (nNOS: 89%; iNOS: 11%) with ANT was detected in isolated mitochondria of WT mice, confirming the expression of nNOS in mitochondria. In contrast, iNOS expression was increased in Cx43^cre/fl mitochondria (iNOS: 90%; nNOS: 10%). The mitochondrial NO-synthesis rate (nmol NO/min/mg protein) was reduced in Cx43^cre/fl mitochondria in comparison to WT mitochondria (Cx43^cre/fl: 0.1±0.015; WT: 0.3±0.018; p<0.001; n=5–7).

Conclusion: 

A reduced mCx43 content is associated with a switch of the mitochondrial NOS isoform and the respective mitochondrial NO-synthesis rate.