The fresh SA node weighed against the latest ventricle expresses calcium delicate adenylyl cyclase isoforms [ 105 ]
50 pS in contrast to the situation in the ventricle where it is closer to 70–80 pS [ 95 ]. ATP channels containing the Kir6.1 subunit. Mice with global genetic deletion of Kir6.1 develop SAN failure and also heart block which leads to sudden death [ 93 , 96 ]. In our own studies we have studied mice with selective deletion of Kir6.1 in vascular smooth muscle or endothelium [ 93 , 97 ]. These mice do not show the rhythm disturbance even with provocations that might provoke vasospasm and suggest by exclusion a potential role for Kir6.1 in the SA and atrioventricular node [ 93 , 97 ].
We have explored this question in the mouse using cre\loxP technology [ 98 ]. We made use of a cre driver line that allows selective deletion of genes within the adult conduction system on the addition of tamoxifen [ 98 , 99 ]. The resulting knockout mice are labelled as cKO. We first isolated KATP currents in single SAN cells and find that only approximately half the cells contain currents [ 98 ]. This is not surprising given the heterogeneous electrophysiological nature of the SAN [ 100 ]. The pharmacology of the response is also interesting with diazoxide activating and tolbutamide inhibiting and this profile is more typical of SUR1 than SUR2 [ 101 ]. In murine atrial myocytes SUR1 underpins a significant population of KATP channels [ 102 ]. Furthermore when action potentials were measured in single SAN cells, there was delayed repolarisation in cKO cells resulting in prolonged cycle length [ 98 ].
The murine model also allows the study of your own into the-vivo emotional outcomes of the Kir6.step 1 removal [ 103 ]. Using implanted telemetry probes, i measured heart rate and ECG details into the awake mice more than a prolonged several months. cKO rats try bradycardic and also in certain mice there were symptoms regarding sinus arrest that happen to be not found in littermate regulation [ 98 ]. Also, there is a sign of atrioventricular nodal dysfunction that have an increase of one’s Pr interval toward ECG [ 98 ]. We also noticed specific small pathological transform which have fibrosis throughout the SA node in certain of the cKO mice [ 98 ]. girlsdateforfree Profiel This new phenotype is not as noticable as with the worldwide Kir6.step one knockout animal although impairment of vascular reactivity and you can use up all your regarding protective solutions many years [ 93 , 98 ].
Significantly this number of cells is actually rather reduced in cKO mice [ 98 ]
In summary, our recent studies show that KATP channels are constitutively active in SA nodal cells. The current seems to influence repolarisation predominantly and this results in an increased cycle length. In-vivo this leads to bradycardia but there was also evidence of sinus pauses, heart block and pathological changes in the SA node [ 98 ]. It is surprising that there are no effects on the maximum diastolic membrane potential. Kir6.1 is a member of the inwardly rectifying family of potassium channel however the currents are more outwardly rectifying [ 98 ]. Additionally, it is plausible that there may be some cyclical regulation of KATP channel activity during repolarisation perhaps by calcium and\or cAMP and protein kinase A to explain this paradox. Signalling via cAMP seems to be significantly adapted in the SA node. Adenylyl cyclase is constitutively active and this leads to basal protein kinase A activation [ 104 ]. PKA activity, and also that of calcium calmodulin dependent kinase II, is necessary for normal pacemaking as inhibitors of these kinases lead to significant slowing of beating [ 25 , 106 ]. Given the sensitivity of both adenylyl cyclase and calcium calmodulin dependent protein kinase II to calcium there may also be cyclical variations in activity that, in addition to phosphorylating phospholamban, may also phosphorylate KATP channels. On the background of high phosphatase activity this may lead to variations in beat-to-beat channel activity and account for the prominence of currents during repolarisation. It is also known that KATP channels can be regulated by calcineurin [ 107 , 108 ]. It is known that Kir6.1-containing channels are prominently regulated by hormonal pathways and protein kinases both in heterologous and native tissues [ 93 , 109 , 110 ].