Slow 'NOT IN' query

SELECT SUM(amount) as amount FROM transactions WHERE account_id = 'L1095' 
AND (trans_date > '2012-08-23 13:37:00' OR (rent_run_id > '2012-08-23 13:37:00' AND trans_type = 5001)) 
AND rent_run_id < NOW() AND trans_type = 5001 AND amount > 0 
AND amount NOT IN (SELECT amount FROM transactions WHERE account_id = 'L1095' AND rent_run_id > NOW() AND trans_type = 5001)

SELECT SUM(tx.amount) amount 
  FROM transactions tx
  LEFT
  JOIN transactions ty
    ON ty.account_id = tx.account_id
   AND ty.trans_type = tx.trans_type
   AND ty.rent_run_id > NOW()
   AND ty.amount = tx.amount
 WHERE tx.account_id = 'L1095' 
   AND (tx.trans_date > '2012-08-23 13:37:00' OR (rent_run_id > '2012-08-23 13:37:00')) 
   AND tx.rent_run_id < NOW() 
   AND tx.trans_type = 5001 
   AND tx.amount > 0
   AND ty.account_id IS NULL;

'1', 'SIMPLE', 'tx', 'ALL', 'account_id', NULL, NULL, NULL, '27425', 'Using where'
'1', 'SIMPLE', 'ty', 'ALL', 'account_id', NULL, NULL, NULL, '27425', 'Using where; Not exists'

delimiter $$

CREATE TABLE `transactions` (
  `idtrans` int(11) NOT NULL AUTO_INCREMENT,
  `account_id` varchar(45) NOT NULL,
  `tenant_id` varchar(45) DEFAULT NULL,
  `trans_id` int(11) NOT NULL,
  `job_id` int(11) DEFAULT NULL,
  `amount` decimal(8,2) NOT NULL DEFAULT '0.00',
  `description` text,
  `trans_date` datetime DEFAULT NULL,
  `trans_period` varchar(16) DEFAULT NULL,
  `rent_run_id` date DEFAULT NULL,
  `trans_type` varchar(45) DEFAULT NULL,
  `payment_ref` varchar(45) DEFAULT NULL,
  `payment_type` int(11) NOT NULL DEFAULT '6',
  `recon_date` date DEFAULT NULL,
  `recon_stat_no` varchar(45) DEFAULT NULL,
  `recon_id` int(11) NOT NULL DEFAULT '0',
  `recalled` smallint(6) NOT NULL DEFAULT '0',
  PRIMARY KEY (`idtrans`),
  FULLTEXT KEY `account_id` (`account_id`)
) ENGINE=MyISAM AUTO_INCREMENT=62353 DEFAULT CHARSET=latin1$$